FNAMTB has an amazing soundtrack you can listen to it on SoundCloud or Spotify with the "Five Red Nights" album, or you can also listen to tracks that never made the cut, due to the cancellation, with the "FNAM: The Leftovers" album on those two platforms as well.
But, there is an definitive edition for the game's OST, and that is the "One Red Soul" album. It combines the two albums together, with some bonus tracks, you can listen to it on BandCamp.
You can listen to it by going to this link here: https://tuli-tetrity.bandcamp.com/album/one-red-soul
There is a secret title screen that can pop up when going to the title screen with a music sounding reversed. It has about a 3% of occurring. The picture features Matt's ancestor, not much is known about her, but more information probably would've been revealed somewhere in Chapters 4 and/or 5.
Two sequels planned even though the original game wasn't even finished yet. These were "Five Nights at Mario's Bloody Eyes" and "Five Nights at Mario's Last Moon." Sadly, once FNAMTB got cancelled, of course the sequels had to go...