I believe Katharina was asking about adding music to videos recently but I couldn't find the thread. Anywho… I used this source for one of my videos today.
http://dig.ccmixter.org/
It wasn't too difficult, even for someone tech un savvy like me. Since I'm a Mac/iMovie user, I just "Save Link As" to my desktop, added the file to a playlist I've created within iTunes to use with iMovie, and then added it to my video from within iMovie. I'm sure there's a dozen other ways to do the same thing but this works for me.
I don't make videos for commercial purposes but they do have some songs available for those who do.
Obviously YouTube has an extensive library of music if you want to go that route.