Need quite a bit of clarification before being able to help with your sound problem. Do you have sound working everywhere except Youtube? Do you have Adobe Flash installed and working properly on other sites? Java?
Sound working ok with system sounds? Can you play MP3 or other sound files without being online? Speakers and headphones working normally?
Which sound card do you have? Run lspci in a terminal and you should see your card listed. You can cut/paste that part of the output here if the sound card itself may be your problem.