Music
Music is a powerful tool in filmmaking. And choosing the right mood is an art in itself. Professional film directors work directly with composers to produce music & sound design that integrates perfectly with the imagery on the screen. They will often choose published songs to connect with the audience adding emphasis to certain scenes.
If you plan on publishing your film online, you must receive clearance to use any music that has been copyrighted (i.e. most commercial music you hear on the radio). As licensing this music is quite expensive, we have several other options available to student filmmakers.
Royalty Free audio allows you to use the music in any way you choose. There are also plenty of Free Royalty Free audio search engines that exist. Often these artists just want their music heard and will make it available through Creative Commons licensing for you to use.
These sources are excellent for free music.
Smart Sound: Royalty Free Music
MobyGratis (account needed)