Have you tried restarting your computer?
Edit: Just went to Youtube to take a look. Couldn't find a "mute" there, but did find a slider associated with the sound icon. Sliding all the way to the left took the sound off; sliding back to the right turned it back on. (This is right below the picture, towards the left of the screen.)
Last edited by Prism99; 04-08-2014 at 12:14 PM.