Double click the speakers from the Sound control panel, and verify this:
On Levels Tab, make sure there are no red circles with lines (turned off symbol) next to any of the items in the list. Make all volumes 100%
On the Ehancements tab, click the box that says Disable all Sound effects, an click the box that says Immediate mode.
On the Advanced tab, make sure both Exlusive boxes are checked. Also, try setting the quality to 16 bit 48 khz.
Also if you have headphones, try them in the same jack just for verification.