Using songs in videos on youtube, "copyrights"?

This is a "loop hole" in the Digital Millennium Copyright Act that is called "Fair Use". You are allowed to use some types of content, as long as you mention that you don't have rights to it. You cannot distribute it in its raw form and you have very specific restrictions, but you can generally use them in videos. For more information, please refer to the Youtube Terms of Service and the Digital Millennium Copyright Act for more info. These have been linked in the sources