Archive.org also has a nice collection of (often free or CC-licensed) "Netlabel" music, much of it electronic.

http://www.archive.org/details/netlabels

I've been involved in developing NAEV [0]. Some of its music is "stolen" from other games under CC license, and some was produced by players.

You can start by borrowing music from other games and put an announcement (on forums, mailing lists, in game) saying that you need music. If people like your game somebody with musical skills will help you creating soundtracks eventually. Just don't expect this to happen overnight. NAEV has been under development for over 6 years.

[0] http://github.com/bobbens/naev

There's also http://www.musopen.com/