it sounds like you need an audio driver..
start menu > Right-Click My Computer > properties > Device Manager >
(if the sound card does not have a driver installed.. it will be shown in the list with a caution mark beside it)
simply Right-Click the sound card & click update driver
if the OS can not update the driver automatically.
you may need to go to the manufacturers website of the sound card to retrieve a driver.
if you know you don't have a external sound card installed.. then go to your computers manufacturer's website & go through & download the proper driver
& also, keep in mind that windows 7 has not been officially released.
so not all drivers for every device present at the moment.
-RandysSOHC