It seems to work in full screen mode too. I don't think the sound gets worse, just a bit different. Give it a whirl.

&fmt=18

very cool!

It works great, and I did not hear any difference in the sound quality.

C juste un essai

Thank's for the french language

You can decide the video playback quality on youtube too:

http://www.youtube.com/video_quality_settings

I've found out the &fmt=18 is the mp4 version. If you want flash it's &fmt=6.