YouTube has a query parameter to play videos in high quality. In order to set a video to play in high quality, you add &fmt=18 to the end of a YouTube url. I'm not sure how mono sound equates to "high quality", but YouTube is doing just that to some of their videos with &fmt=18. Sacrificing audio quality for video quality doesn't seem to me to be an acceptable trade off.

You can find evidence of this clearly by listening to the following video with headphones on. It's the video by The Fixx "Saved By Zero", uploaded by Universal Music Group.

http://www.youtube.com/watch?v=swzK7Q8teSM

If you're paying attention, you can hear the right speaker is delayed by about about 50ms. You'll also hear a clear separation of left and right channels because it is playing in stereo.

Now, the same video playing in high quality:

http://www.youtube.com/watch?v=swzK7Q8teSM&fmt=18

You can now clearly hear the delay without question because the audio is playing in mono so the delay is now playing equally in both speakers. Another tell-tale sign of mono is that the music sounds like it's coming from the center, not with the stereo spread like the "low quality" version.

Click the link on the bottom right corner of the video "watch in normal quality" to toggle back and forth between the two if you have trouble hearing it the first time.

YouTube videos playing in mono when set to high quality == Lame.