You've not stated which browser you're using. I'll assume that it's Internet Explorer.
Go to Tools > Internet Options.
Click on the 'Advanced' tab.
Scroll down to the 'Multimedia' section and look to see if there's a tick alongside 'Play sounds'.
If there's no tick, click to add it. Then click 'Apply' and 'OK'.
If there's already a tick there, click to remove it. Click 'Apply' and 'OK'. Then go through the process again to put the tick back.
If that doesn't work (and simply restarting your PC doesn't fix the problem), try using System Restore.
Chris