All most of the iDOLM@STER music:
MP3 (updated September 16th 2015) or FLAC (updated September 16th 2015).
If there's anything missing from there, it'll most likely be below, or on the Million Live/Cinderella Girls pages.
More stuff comes out all the time, so keep your eyes and ears open.
Idolm@ster Music Collection (MP3)
http://bakabt.me/161615-the-idolm-ster-idolmaster-music-collection-mp3.html
Idolm@ster Music Collection (FLAC)
http://bakabt.me/159851-the-idolm-ster-idolmaster-music-collection-flac.html
Another site (Contains Perfect Idol 1 and 2)
http://www.anime-mp3.com/2011/07/idolmster-ongoing.html
me to me (Manami Numakura & Yumi Hara) (Also includes Talk tracks)
http://www.mediafire.com/?8bjrh69po5mtcm4 (LQ MP3)
http://www.mediafire.com/?6k4x5qa64a1ckyi (FLAC)
iM@S Ringtones
http://www.mediafire.com/?lodiyhryw9wi2
http://www.mediafire.com/?um5fd238zem9v
http://www.mediafire.com/?3ioc4matb9ylde0
http://www.mediafire.com/?csf3uj24sfbnn
http://www.mediafire.com/?6s4c15c553k3a3s
http://www.mediafire.com/?gds0smc5z0481iv
http://www.mediafire.com/?8gexjk214c8a41k
http://mfi.re/?56edj55ng1hs7uv
http://mfi.re/?ej7jct65gqayvcy
Aoi Tori Sheet Music
http://www.mediafire.com/?dw8eh4ga74ugm3g
Perfect Idol 03
FLAC
http://www.mediafire.com/?p3hl0hd81k38fr3
MP3
http://www.mediafire.com/?r33sg6oqsfhvgqs
Perfect Idol 04
http://www.mediafire.com/?q4cidfvwcd0014r
http://www.mediafire.com/?rm52piha1n2ohlj
8BIT iDOLM@STER Best Album (The compilation of previous 5 albums in 3 disc)
http://www.mediafire.com/?kxpwm2j8o3o86wk
http://www.mediafire.com/?n2ctb4fftiquj4g
http://www.mediafire.com/?9fxvqu8fzr900mo
THE IDOLM@STER Station!!! Second Travel -Seaside Date-
(Password is "De drie kleine biggetjes" without quotes.)
http://www.sendspace.com/file/qqki0x
MASTER BOX VII
(MP3) http://www.mediafire.com/?x0iuxp0iwbp0n
(FLAC)
http://www.nyaa.eu/?page=torrentinfo&tid=376994
THE IDOLM@STER STATION!!!! Amazing Grace
(320 MP3 and FLAC)
http://www.mediafire.com/?c4q313nj3hgsd
(FLAC Only)
http://www.nyaa.eu/?page=torrentinfo&tid=382620
Second Vision Yukiho Singles Pack
http://www.mediafire.com/?i34krw66jlz1068,77zsyyur0uica57 (Old, larger version)
http://www.mediafire.com/?k82z1goqxkihe5g (New, consolidated version)
Winter Festival of Music live performance audio rips:
http://mediafire.com/?a296uk19ay98k
YuruYuri/Kannagi cover CD
http://www.mediafire.com/?m9819s66whtfnd4 (MP3)
http://www.mediafire.com/?cewpyq7wzaijxm8 (FLAC)
YAYOI & YUKIHO COVER -SUMMER SONGS-
MP3: http://www.mediafire.com/?w87ofm42uz2l2xs
FLAC: http://www.mediafire.com/?ncqed3mc44td4i1
Scans: http://www.mediafire.com/?tadt5b990ixjty2
MAKOTO & HIBIKI COVER -SUMMER SONGS-
MP3: http://www.mediafire.com/?t5nkorc6xon9bx1
FLAC: http://www.mediafire.com/?dj264rfdk4j8k7h
AMI/MAMI & AZUSA COVER -WINTER SONGS-
MP3: http://mfi.re/?zu5l4n9aj8jj7wz
WAV+CUE: http://www.nyaa.se/?page=view&tid=527194
HARUKA & CHIHAYA COVER -WINTER SONGS-
MP3: http://mfi.re/?hd5oa3wpjdt5seq
WAV+CUE: http://www.nyaa.se/?page=view&tid=527195
MIKI & TAKANE COVER -SPRING SONGS-
IORI & RITSUKO COVER -SPRING SONGS-
SUMMER VAC@TION REMIX BGM CD
Preview: http://www.mediafire.com/?5nq2pokumdrh16j
Full: http://blog.7thwraith.net/2013/08/25/the-idolmster-movie-vaction-pack/
THE iDOLM@STER STATION!!! FAVORITE TALKS
http://www.nyaa.eu/?page=view&tid=459400
ANIM@TION MASTER READY!! Solo Remix
http://www.nyaa.eu/?page=view&tid=459399
ANIM@TION MASTER CHANGE!!!! Solo Remix (thanks Mycstea!)
MP3: http://mfi.re/?hq346kk1dq17l4m
FLAC: http://mfi.re/?q99jm7lrifimb
MUSIC♪ (M@STER VERSION)
http://mfi.re/?c65ptldc7k5bbts
Visionary (M@STER VERSION)
Vault That Borderline! (M@STER VERSION)
765PRO ALLSTARS+ GRE@TEST BEST! -THE IDOLM@STER HISTORY-
MP3: http://www.nyaa.se/?page=view&tid=475322
FLAC: http://www.nyaa.se/?page=view&tid=475320
765PRO ALLSTARS+ GRE@TEST BEST! -SWEET&SMILE!-
MP3: http://www.nyaa.se/?page=view&tid=487602
765PRO ALLSTARS+ GRE@TEST BEST! -COOL&BITTER!-
MP3: http://www.nyaa.se/?page=view&tid=494787
765PRO ALLSTARS+ GRE@TEST BEST! -LOVE&PEACE!-
MP3: http://www.nyaa.se/?page=view&tid=507152
Movie theme song "M@STERPIECE"
MP3 (Disc 1 only): http://mfi.re?g3dj3j44aax0jbh (ripped by Hikari no Akari)
FLAC (Disc 2 only): https://mega.co.nz/#!s9IzxCjY!Q4OKwcUrzWbvH9kNmAopn4gShmi8AqY7vbn6oBznQw8
FLAC + BDMV (Disc 1 and 2): http://www.nyaa.se/?page=view&tid=517848
Individual FLAC + MP3 (Disc 2): http://mfi.re/?xoxw4onxakyyf
The iDOLM@STER Movie: To the Other Side of the Sparkle! OST
MP3: http://www.nyaa.se/?page=view&tid=520941
FLAC: http://www.nyaa.se/?page=view&tid=520940
M@STERS OF IDOL WORLD!! 2014
MP3: http://www.nyaa.se/?page=view&tid=525596
FLAC: http://www.nyaa.se/?page=view&tid=525555
The iDOLM@STER Movie: To the Other Side of the Sparkle! insert song single ("Ramune Iro Seishun" and "Machiuke Prince" M@STER versions)
MP3: http://www.nyaa.se/?page=view&tid=565954
FLAC: http://www.nyaa.se/?page=view&tid=565953
"The Remixes Collection THE IDOLM@STER TO D@NCE TO" remix album
MP3: http://www.nyaa.se/?page=view&tid=582740
9th ANNIVERSARY WE ARE M@STERPIECE!! 自分REST@RT&SMOKY THRILL solo album
MP3: http://www.nyaa.se/?page=view&tid=582780
To the Other Side of the Sparkle! ending song single ("Nijiiro Miracle" and "Fate of the World" M@STER versions)
MP3: http://www.nyaa.se/?page=view&tid=585696
FLAC: http://www.nyaa.se/?page=view&tid=585694
MASTER SPECIAL SPRING LIMITED EDITION album (solo versions of "Matane")
http://mfi.re/?d9dw2w367f06kk0
MASTER ARTIST 3 Prologue ("ONLY MY NOTE" and "Acceleration" M@STER versions)
MP3: http://www.nyaa.se/?page=view&tid=592992
FLAC: http://www.nyaa.se/?page=view&tid=592991
THE IDOLM@STER STATION!!+ "Monday Night Fever☆" album
MP3: http://www.nyaa.se/?page=view&tid=604807
PERFECT IDOL THE MOVIE
MP3: http://www.nyaa.se/?page=view&tid=605184
FLAC: http://www.nyaa.se/?page=view&tid=605183
The 9th ANNIVERSARY "We Have A Dream" + "Curtain Call" SOLO MIX album
MP3: http://www.nyaa.se/?page=view&tid=611497
"IDOL POWER RAINBOW" CD single version
MP3: http://www.nyaa.se/?page=view&tid=617097
THE IDOLM@STER ORIGINAL CD SET -1111 BRIGHTS and MAGICS-" (featuring Haruka singing "Onegai! Cinderella" and Uzuki singing "M@STERPIECE")
LOW QUALITY RIP (disc 2 only): http://mfi.re/?0hnj4ehgi0duprg
HIGH QUALITY RIP: http://www13.zippyshare.com/v/jjK2PTcl/file.html
MASTER ARTIST 3 01 Haruka:
MP3: http://www.nyaa.se/?page=view&tid=681387
MASTER ARTIST 3 02 Hibiki:
MP3: http://www.nyaa.se/?page=view&tid=681388
MASTER ARTIST 3 03 Makoto:
MP3: http://www.nyaa.se/?page=view&tid=681389
MASTER ARTIST 3 04 Miki:
MP3: http://www.nyaa.se/?page=view&tid=697611
MASTER ARTIST 3 05 Iori:
MP3: http://www.nyaa.se/?page=view&tid=697612
MASTER ARTIST 3 06 Takane:
MP3: http://www.nyaa.se/?page=view&tid=697614
MASTER ARITST 3 "99 Nights" remix version
MP3: http://mfi.re/?58ql57c5s4oc1wl
MASTER ARTIST 3 01-06
FLAC: http://www.nyaa.se/?page=view&tid=705058
MASTER ARTIST 3 07 Chihaya
MP3: http://www.nyaa.se/?page=view&tid=709127
MASTER ARTIST 3 08 Mami
MP3: http://www.nyaa.se/?page=view&tid=709129
MASTER ARTIST 3 09 Yukiho
MP3: http://www.nyaa.se/?page=view&tid=709130
MASTER ARTIST 3 "Shizuka na Yoru ni Negai o..." remix version
MP3: http://mfi.re/?zid0bzj4dxg6401
The M@STERS OF IDOL WORLD 2015 theme song "Ai MUST GO"
http://www.nyaa.se/?page=view&tid=702757
http://pan.baidu.com/s/1pJ9kTWB + 5whp
M@STERPIECE & 10th Anniversary Mix
https://mega.co.nz/#F!B1Y2lZyI!YF_j3CX2fGzDc4WeRhaLdg
THE IDOLM@STER STATION!!+ WINTER MEMORIES
http://vn-sharing.net/forum/showthread.php?t=11765
MASTER SPECIAL SPRING -Limited Edition-
Capellas/Mixes
http://www.youtube.com/user/IdolBlueChihaya/featured
http://www.youtube.com/user/yuekana
Haruka Chihaya overmaster duet:
http://www.mediafire.com/?wz5orgdos66tubq
Iori, Yayoi, Chihaya Brand new day trio:
http://www.mediafire.com/?648irzr4z5pneoh
Chihaya Ready & Change special edition solo:
http://www.mediafire.com/?yqz3igys48z4wn4
Chihaya and Ranko duet version of "Tsuki no Shizuku"
http://mfi.re/?ut7q763t8buplfq
Assertive’s RANDOM STUFF – PON DE SNAIL (Snail’s House x iDOLM@STER) remix
https://soundcloud.com/afrandom/pon-de-snail
https://mega.co.nz/#!wRpRnYDb!NIMbrjNyPW-ynpGnECiWWCX2np-iq1CQT0txkb0FApk
Chickari's "Hotel Moonside Kari Remix"
ESTA*'s "GOIN'!!! (ESTA Remix)"
https://soundcloud.com/zawa_s/goin_rmx
Etc
iM@S cover song list:
http://imas.maruamyu.net/song/cover.html
First Step
Show Time!
Super☆Star
(Some covers of Vocaloid songs.)
http://www.mediafire.com/?t968d468wdmvhm8
320 kbps MP3 version of Break Volume 4
http://www.mediafire.com/?kbhefp55hgf889n
(Doujin CD) "Sweet Heart" by Digital Oni Sound (contains some iM@S music rearrangements) http://www.mediafire.com/download.php?tv5clrvk58viaic
Relaxing/uplifting songs from Chihaya and Haruka
http://www.nicovideo.jp/watch/sm15385866
http://www.nicovideo.jp/watch/sm13124029
http://www.nicovideo.jp/watch/sm7535400
http://www.nicovideo.jp/watch/sm15136463
http://www.nicovideo.jp/watch/sm15858076
http://www.nicovideo.jp/watch/sm16499413
Vocal-Boost Little Match Girl
http://www.mediafire.com/?sj4h01vppa02ajj
"AEIUEOA!" sung by Haruka and Chihaya
http://www.mediafire.com/?iqsvreo14944izm
(Transcribed lyrics)
Master Box VIII
http://www.mediafire.com/?i33scj0tay3dz4v
http://www.mediafire.com/?07z00c8dpwfccv7
Chihaya's version of Mog's Theme from Final Fantasy VI OST
http://www.mediafire.com/?a1aj6qwz4d4y4il
Kosmos, Cosmos acapella mix
http://www.mediafire.com/?tnz587mbweca5hd
Machiuke Prince in-game rip
http://mfi.re/?1d3reudkg9yeo9e
First step instrumental game rip
http://mfi.re/?9i2ae6k6628nkge,z9kc7gb4fagex4c
G4U compilation mixes
Shiny TV instrumental music rips
Subbed Hibiki songs
Fan-made Hibiki M@STER version of "shiny smile"
v1: http://mfi.re/listen/d36txu6rachz2bo
v2: http://www.mediafire.com/?iyzj8dapy7myrft
Shisotex's iM@S freefunk colleksi [A] and [B]
https://www.mediafire.com/?nv88o5y6e63sp1t,is93ns672pe76kp
"Welcome!!" instrumental version (from the Million Live 1st LIVE BDs)
http://mfi.re/?tqdsxwnpvhkql4v
Namassuka Special
Namassuka Special 01 (featuring Project Fairy and "Kyun! Vampire Girl)
MP3 320kbps: http://www.mediafire.com/?hnp0r3erh0u015u
FLAC version: http://www.mediafire.com/?bm27n18sl4fvo
"Hatsukoi ~Isshou Kataomoi no Sakura~" (from above) has been transcribed via OCR: http://pastebin.com/q80jESnn
Namassuka Special 02
MP3: http://www.nyaa.eu/?page=view&tid=363494
FLAC: http://www.mediafire.com/?2on07g7wnsj1gfc
A capella of the M@STER version of Nanairo Botan
http://www.nicovideo.jp/watch/sm18813886
Namassuka Special 03
http://www.mediafire.com/?jh1l44gtzie9lo4
Namassuka Special 04
MP3: http://www.mediafire.com/?it4y6m64kkfgbx5
FLAC: http://www.mediafire.com/?o19bei9d12xajoa
An Acapella of "Honey Heartbeat"
http://www.mediafire.com/?36b1k3ya7umpx6a
Namassuka Special 05
MP3: http://www.mediafire.com/download.php?81zmjv8l8v5kf93
FLAC: https://mega.co.nz/#!fclklKJB!Mz8PUy4dvehfDQH0Qnk5qmxKOAAF7s1xZ9GVTDsgJts
Curtain Call
FLAC:
http://www.mediafire.com/?9214d123oe8bbhy
MP3:
http://www.mediafire.com/?ca59bsur4mdmk0a,mpkcrlfkcca3y8b
Namassuka Special String Quartet Hatsukoi Musical Suite
http://www.mediafire.com/?61ffv0x07814f30,sybs2cw65hdsr6w (MP3)
http://www7.zippyshare.com/v/35712283/file.html (booklet scans)
First Step (M@STER VERSION)
http://www.mediafire.com/?ci4sj02u7sjf64y
My Wish
Menelkir's nine-part multi-torrent iDOLM@STER music compilation (August 15th 2015)
http://www.nyaa.se/?page=search&term=[Menelkir]+THE+iDOLM@STER+Music
https://archive.moe/vg/post/112211461
http://www.nyaa.se/?page=view&tid=723059
Compilation Tracks
(The blue button that says mp3 is the download link.)
Iori
http://nicosound.anyap.info/sound/sm12219866
Ami and Mami
http://nicosound.anyap.info/sound/sm9783585
Makoto
http://nicosound.anyap.info/sound/sm945931
Chihaya
http://nicosound.anyap.info/sound/sm12103265
Yukiho
http://nicosound.anyap.info/sound/sm11545624
http://nicosound.anyap.info/sound/sm12143075
Ritsuko
http://nicosound.anyap.info/sound/sm12306968
Miki
http://nicosound.anyap.info/sound/sm12220155
Azusa
http://nicosound.anyap.info/sound/sm12307003
Yayoi
http://nicosound.anyap.info/sound/sm12380758
Haruka
http://nicosound.anyap.info/sound/sm12219814
Hibiki
http://nicosound.anyap.info/sound/sm9287501
Takane
http://nicosound.anyap.info/sound/sm9282891
All idol solo simultaneous versions of LOST
http://www.mediafire.com/?w31a838i6fgw962
NoNoWire rip
http://rip.braindown.net/mogra1/2012/
Shisotex's set on NoNoWire
http://www.mediafire.com/?1f7yqj8aa9jml6m
A pack of Yukiho solo performances from iDOLM@STER 2
http://www.mediafire.com/?65yy9d1pd86dx64
Master Book Scores
http://www.mediafire.com/?1625e14h180qy
Kuroi sings
Kasumi sings
Hamuzo sings
Chousuke sings
Yurishii's Dream, her last song
http://www.nicovideo.jp/watch/sm11341898
Yurishii's Birthday Commemoration Music Medley
http://www.nicovideo.jp/watch/sm19505291
Azumin and Yurishii's My Song
http://www.nicovideo.jp/watch/sm19532223
The Famison 8BIT☆IDOLM@STER BEST ALBUM + LIVE DVD
(3CD FLAC + concert video): http://www.nyaa.eu/?page=torrentinfo&tid=385750
http://www.nyaa.eu/?page=torrentinfo&tid=385759
Ripped songs from iM@S STATION (Danketsu and Kamisama no Birthday)
How to get all most of the iDOLM@STER music (depreciated):
1. Get the Music Collection torrent in FLAC or MP3 (MP3 is recommended as it has a few extra albums).
2. Get Namassuka Special 02, 03, 04, 05, Curtain Call and String Quartet.
3. Get Twelve Seasons 01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 11 and 12.
4. Get Cinderella Master 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, Onegai! Cinderella, Kagayaku Sekai no Mahou and Cool Jewleries! 001.
5. Get LIVE THE@TER PERFORMANCE 01, 02, 03, 04, 05 and 06.
6. Get the YuruYuri/Kannagi cover CD, the MAKOTO & HIBIKI and YAYOI & YUKIHO Summer Song cover CDs, and the READY!! Solo Remix CD.
7. Get 765PRO ALLSTARS+ GRE@TEST BEST! -THE IDOLM@STER HISTORY-.
If you want all of the above and the live concert videos as well, get this torrent: http://mfi.re/?dubwexjsw4ccyxt
There's a LOT of iDOLM@STER music out there, but this will get you most of it. Beyond that, you're on your own. Good luck.