Can you hear sound otherwise, eg playing music from music tracks on your computer and not from within a browser?
If so then there is nothing wrong with the computer sound.
So, just guessing here, but when you are on the internet watching a video or listening to music, look for the "Volume Control" on the media player that you can see in the browser, and see if you need to increase that.
90% of the time i have no sound when i am on the internet. some days i have sound when i am online i use inter explorer,firefox. google chrome i ran a malware scan and a virus scan find nothing. can anyone please help me