Look at the bottom of your screen at the left hand side, its called a task bar, u should be able to see lots of little icons. Look for one that looks like a megafone click on this, if there is a tick in the box its muted, just take the tick of by clicking on it, then drag the volumiser button up.
if you have a web or Google accelerator try to deactivate it, it might be blocking your audio or try restarting your computer. Srry, thats all i know, hope it helped.
it could be on mute, or the volume could be turned down, whether it's on the actual computer or the speaker. the speakers could be turned off, or unplugged.
Your speakers are probably not plugged in tightly. Replug them in. If YouTube doesn't have any sound, you may have to download the newest Adobe Flash available. Otherwise, get a new speaker