I have many videos on YouTube with music in them that are copyright violations. When you upload it, YouTube has software that somehow is able to recognize copyrighted music, you can either have YouTube remove the music (Silent Video) or allow the copyright to receive any and all monetary gain that video receives. 

I myself think it's kind of a crock in a way, as the video you create is also copyright so to speak, as long as it's 100% original content. It really should be split 50%, but the music people have way bigger pockets, so they get their way I guess.

Just don't use copyright music on a video that has the potential to go viral, or you won't see a dime off it...

YouTube does have a large library of music you can download for free and use without any copyrights, or you can search any song to see if it has a copyright against it before using the song or uploading it.

Hope this helps...