I have been doing a little bit if checking into this, and I am beginning to think that it might have to do with either how well the browser supports HTML5 (full/partial implementation), or how well youtube adheres to the HTML5 standard.
To summarize the reports I am seeing here:
Firefox: has no issues with using HTML5 on youtube,
Chrome (on linux): looks like no issues here either
Chromium: missing the sound but has video
Midori: missing the video but has sound
This will require a bit more looking into, but as I said earlier the problems appear to be browser-dependent rather than OS dependent. This seems to imply that youtube is still not fully compatible with HTML5 (I think it still warned that it was experimental when you convert over), or some other related issue.
~ Ken Moore ~