You can get sued if you aren’t careful and you use music for commercial purposes without a license. Just ask Tony Romo’s (QB of the Dallas Cowboys) wedding photographer, Joe Simon. He used a Cold Play song in a wedding slide show video that went viral on youtube and he had to settle with Cold Play for an undisclosed amount. The full PDN article is here.
Moby has a website called mobygratis.com that offers “film music” for “independent and non-profit filmmakers, film students, and anyone in need of free music for their independent, non-profit film, video, or short.” If you want to use the work for commercial purposes, you have to e mail and pay for that use. I have used mobygratis for a behind the scenes video and was granted permission to do so.
Another option is triple scoop music.