I hope you are joking.
Its illegal to redistribute copyrighted works, such as music without the creator's permission which generally involves royalties ... this includes incorporating even part of one work with an original piece (like a commercial artist's song with your animation).
Artists and the RIAA haven't been doing squat about it, they're more focused on destroying P2P and MP3s it seems, but legally they could take newgrounds down or at least all of the animations it contains that include copyrighted music.