Perhaps this has been going on for a while, but I've never noticed it before. YouTube users often create an original video using their favorite popular song as the audio. I'm afraid that they won't be able to do that much longer, since YouTube has started muting videos that use unauthorized copyrighted music (and that pretty much means all user-created videos.)

You can see some examples here, here and here.

The official notice from YouTube under the video says the following:

"This video contains an audio track that has not been authorised by all copyright holders. The audio has been disabled."

If YouTube starts being thorough about this, you can expect to see a significant percentage of all YouTube videos muted. The implications are a bit different than with removing copyrighted professionally produced content, like an official music video; we're talking about tens of thousands of fan made videos, funny spoofs, remixes and the like being pretty much destroyed, and I'm guessing users will be less than thrilled about it.

Thanks to Aniq Rahman for the tip.