YouTube Begins Muting Copyrighted Music
It's unclear when this began, as they didn't announce the policy change. Besides the obvious muting of the video, below the video you will see the message:
This video contains an audio track that has not been authorized by all copyright holders. The audio has been disabled. More about copyright
Naturally you won't see this on an embedded video, just on the actual YouTube video page. Here's an example (from above). If you look at the comments in the videos which are muted, you can see the new policy is not popular (and that's an understatement). And realistically, if they're going to mute the video, why not just pull it? Why keep it up at all? The only example we could think of where it would be nice to keep the video up would be something like the "Dancing Baby" video which was pulled because it had Prince music in the background of an actual baby dancing.
Despite all this, it's far easier to find copyright-infringing videos that are not muted than those that are. Still just in case you find you can't hear anything on a video, don't fiddle with your speakers, audio card, headset, or whatever, until you look for that copyright message.