It seems YouTube has started muting user-generated videos that use unlicensed music.
A writer on Mashable has found a number of such videos that have the YouTube disclaimer "This video contains an audio track that has not been authorised by all copyright holders. The audio has been disabled".
It mainly seems to affect user generated content that samples music owned by Warner Music who, as previously reported, are currently in dispute with the video service over licensing arrangements.
This presumably means YouTubers are no longer able to upload videos containing music not available in its original form elsewhere on the site (technically speaking I'm not sure they were ever meant to).
When all four majors are signed up to YouTube that won't effect so many user-generated videos - but once a catalogue the size of Warner Music / Warner Chappell is removed it will impact on many user-generators.