music wont play from youtube or any other internet source, but in the itunes library it works..

any time im using the internet wanting to play videos or listen to music the sound doesnt seem to work. i can turn up the volume and hear the clicks from the volume bar but the sound will not play!! and it works anytime im in itunes store or my own library. ( used about 4 diferent sets of headphones, so this is not the cause.)