YouTube has implemented a new policy on videos that contain unauthorized, copyrighted music: They’re muting them.
Want to see (but not hear)?
If you go to YouTube and look at a page with a muted video, they have this written beneath the video:
Oddly enough, enforcement seems to inconsistent (although I’m sure this video could be muted any day now):
That’s Dream Theater, the same band as the first video, the song is different, but it is from the same concert DVD, Score.
Anyway, I just figured that you should be aware so that you aren’t completely puzzled by a YouTube video that has no sound.
The Geek Whisperer 
Well, I noticed and wondered why the video ran better w/out the audio. As for inconsistency, it’s like editing a manuscript– sometimes, an error slips from the editor.