Audio issue with YouTube and Myspace

This is a new issue for me. I've had this computer for 2 years now and until recently I've been able to view videos and hear songs on various websites no problem. However, in the past few months it's stopped working. On YouTube I can see the video but there is no sound. On Myspace I can see a song is playing but I can't hear it. Oddly enough, Quicktime works just fine playing videos and sound, and my iTunes plays just fine. I feel like I've checked every sound control I can on my computer, so I really don't understand why I can't fix this. Any advice or help would be much appreciated. Thanks!