No one can really say for sure when this started, but YouTube has been muting videos that have copyrighted music used without permission (typically fan videos). The videos will play like normal, but there will be no audio, and a message that says, "This video contains an audio track that has not been authorised by all copyright holders. The audio has been disabled."

YouTube is claiming that this is an improvement, since they used to just erase the videos, but are now giving users the option to keep the video and ditch the audio. I'm not sure that this is an improvement, since audio is (somewhat) important to a music video making any sense at all. YouTube might as well be deleting all the clips.[Mashable]