These are a few sites that offer music that you can use for free with video projects.  How the authors need to be attributed can vary with each site – so check out the Legal bits before using anything!  7SXEW5Y6C6XG

http://free-loops.com/
http://www.flashkit.com/loops/
http://www.partnersinrhyme.com/
http://www.newgrounds.com/audio/
http://www.freesound.org/
http://incompetech.com/m/c/royalty-free/