Well YouTube is a video streaming service, so it tries to make the files as compact as possible for a fast viewing experience without pauses for rebuffering.
However, YouTube offers a 'auto quality adjust' depending on the speed of the user's current internet connection and decides which version of the files it will open. The higher quality FLV's (and apparantly with stereo) or the minimized versions. People with a YouTube account can override these settings in their preferences panel if they wish.
To be honest, there's something dead wrong with YouTube's auto-adjuster, at work the internet is no slouch at 600 Kbyte/s download but it chooses to run vids in low quality, at home I can get a booming 8000 Kbyte/s (fibre-glass) and it will run at the high quality, altough a connection of 200 Kbyte/sec would suffice (according to my mad calculations).
Just something you have to accept. Hey, it's a free service anyways, don't complain.

You could add this in the video's notes though if you wish your viewers should hear it the way it was intented to sound (and I totally understand why you'd want this in the first place).
"...and for 3 quarters of it, I had a lunatic trying to get up my arse." - nealmac, Dec 18 2009, 02:46 PM
horrid soundscapes for horrid people.