most often it is a sound driver that is missing.
Open your device manager:
right click my computer
select the hardware tab
and click the button that says "Device Manager"
Check for an item in this list that has a yellow icon with a ! in the middle, likely there will be at least one associated with your audio.
double click it, and a new dialog should open.
under the driver tab there should be an option to install or update driver, choose that and put your windows cd in the drive and be sure to include that drive in the search. Should take a minute or so and Windows will re-install the sound drivers.
If you need more help, do not hesitate to post