iMac - Sound problem with safari and firefox...

for some unknown reason my browsers have no sound when it comes to sites like youtube, dailymotion etc, i tried testing on the apple site with the iphone promos and could hear everything fine. Also can hear everything regarding itunes, adium etc, has anyone ever experienced this problem before and could someone help my out, as its really bugging me thanks


also before you ask, the volume was up on youtube, im not that silly hehe