When you say that the device drivers are working, how did you confirm this? Was it through device manager?
Assuming you are using a sound card, the first thing I would do is make sure that it is seated correctly in the slot - cards can get loose on the rare occasion. If you do that, just make sure you discharge any bodily static first.
Then I would download and install the latest drivers from the manufacturer (if its onboard, download from mobo site).
Hope it helps a little!