Basically Creative Commons provides a few easy licenses that users
can choose from. Usually you only need to provide "attribution" or credit
to the particular artist in your film's credits and you're good to go.
No contacting and signing necessary - it's all already legal.
Kevin MacLeod 's incompetech.com
http://incompetech.com/, License: Creative Commons Attribution 3.0
Probably one of the most popular and best free film soundtrack download sites on the internet. Kevin MacLeod produces and records all of the music on the site which hosts hundreds of tracks for complete free download under the Creative Commons Attribution 3.0 license. Use his music and just credit him (he does appreciate donations as well).
Moby's Moby Gratis
http://www.mobygratis.com/, License: Own - on site (free for non-commercial)
Yes that's right - the legendary Moby is providing some of his own tunes for use in indie/student film projects. Sign-up is required and then you can browse some 100 soundtracks and make your choice. You must submit your request beforehand (which may take some time) and accept their terms of use but it's all worth it for the licensed high quality music.
Free PD
http://freepd.com, License: Public domain
Yet another site from Kevin MacLeod but this time around the music is from a variety of artists and all tracks have been donated to the public domain - meaning no strings attached at all. Very nice if you want to make something small with no space for credits.
Free Music for Filmmakers (Rosie Catalano)
http://sites.google.com/site/rcmusicforfilm, License: Mostly Creative Commons Attribution 3.0
Not a wholly original site title but still useful. Besides the host (Rosie Catalano) a few other artists have also contributed to the this small but unique collection.
CC Mixter
http://dig.ccmixter.org, License: Variety of Creative Commons
A large site listing many different compilations under many different Creative Commons licenses. It's very easily searchable and no sign up is required to download.
Soundcloud Creative Commons
http://soundcloud.com/creativecommons, License: Variety of Creative Commons
The Creative Commons section of the popular Soundcloud site has a fantastic search feature. No sign up is required to download. Unfortunately the site may be a bit buggy on some browsers.
Taylor Howard
http://taylorhayward.org/, License: Public domain ("no copyright")
A few ambient classical piano pieces are available to download and they have "no copyright"... it would still be nice to credit the guy though.
Free Music Archive
http://freemusicarchive.org, License: Variety of Creative Commons
Another large site listing many different compilations under many different Creative Commons licenses. Not as easy to search as Soundcloud and sign up is required to download.
Jamendo
http://www.jamendo.com/, License: Variety (including Creative Commons)
Yet (again) another large site listing many different compilations. Be sure to navigate to the Creative Commons section...
Sonnyboo Free Music
http://www.sonnyboo.com/music/music.htm, License: Public domain
A few tracks are available for free download and are said to be "a donation to the public domain" although credit is appreciated.