If you still have the problem...maybe we can narrow down the factors.

So you can hear the windows notification sounds correct?
Like CD music & online music /radio stations or any website that has a clip or sounds?

Is it only the you tube videos on the you tube website that you can't hear?
or all videos online = Google videos etc?

like what about the hulu website shows, can you hear those?

maybe ??
http://www.google.com/support/youtub...n&answer=58132