I've been listening to music via youtube.com and I came across this little string to add to a youtube's URL to make it a) high quality (if it's available) and b) stereo (if it's available). Copy this: &fmt=18 and paste it to the end of the youtube URL in the browser address bar (then press enter to reload the page with the new settings) and the video will play the best it can be.
You can check out an example: youtube's default and compare it to this one with &fmt=18.
Note: The HQ/&fmt=18 version will be a larger file, bigger screen size and have audio in stereo @ 44 KHz as opposed to mono 22 KHz. Just so you know the guts of the situation.