Are you using Windows Vista?
A feature that is different between Vista and previous operating systems is the way it manages the sound for each application. Vista allows you to set a separate volume limit for each program like Media Player, FireFox, Internet Explorer, etc. While playing a video on youtube right click on the speaker icon next to the time and select "Open Volume Mixer". In the pop-up window check the volume limit for each application is not zero and is not muted.