For some reason, my volume on YouTube has become stuck. Every time I go to YouTube, the mute is on and the volume is slid all the way down to zero. I can adjust it but it only works as long as I keep YouTube open. If I close it and go back, the volume resets to silence.
What browser are you using? Do you have any extensions or addons to modify youtube/video/flash/html5 players? Is there another tab with a youtube video open that you muted earlier? If you open a youtube page in another browser, do you see the same behavior?