Went through the same scenario just today - reinstalling over the existing driver didn't seem to work, as the posters have noted. What corrected the issue for me was to go to Control Panel - Uninstall a Program - then select the original Audio driver from the list and uninstall it. Once it has been removed, go ahead and reinstall the driver (note here - the Zip file on my drivers CD was corrupted and didn't install correctly, but downloading the file from Dell's Support site worked properly). You should see a command window open and a message within saying that it is Restarting Audio Services. Once this occurs, you should see the little speaker icon next to the time lose the red "X", and display the )) that indicates working audio. Restart the system when prompted, and your audio should work properly once again.

Hope this helps some other folks!