No sound for my internet

Hello,

im not sure why, but whenever i try to play a video over the internet, the sound does not work. i know its not my speakers because my iTunes works, and its not the internet connection because when im on a different server the problem is still the same. skype wont even let me initiate a call, and youtube videos cannot be heard. this is the same for all of my web browsers, but i primarily use google crome.
i also have an acer, aspire, 6930.

thanks in advance.