It is possible to mute your sound. You might check for that.
You might also make sure that the speakers are plugged into the correct outlets. I usually figure out which ones are correct by trying them all and stopping when I hear something [img]smile.gif[/img]
To test, I go into Control Panel, Sounds. Then I pick an event with a sound associated and click the test "play" button until I'm satisfied. It's a really quick way to test on demand, and if it doesn't work there, it won't work anywhere...
You might also try a Windows repair install. Your comment that the sound icon suddenly disappeared makes me think of that... could be useful.
Good luck.