Sound from internet not working??

Hello all:

So...the audio from SOME sites on the internet all of a sudden stopped working on my iMac...like Youtube (and other video sites). However, stuff from Apple or ABC, etc work just fine. Anything non-internet works fine. Anyone know how to fix this? I know it's just the computer because the sites that don't work on my iMac work fine on my laptop and at school computers. I have checked my preferences numorous times for anything to do with sound...I have Googled it, and have not found the answer yet

Any help will be much appreciated and I will be forever grateful

Thanks guys,
-Jess