Hi. I have had the same problem. I tried all the drivers mentioned, and those in the Asus Website.
My client had "Multimedia Audio Device" without a driver in the device manager. The Driver does seem to be accepted by the system, but sound still does not work. It is not speaker-related, because ControlPanel -> Sound&Audio shows no sound device present. Also, it is not Disabled from BIOS either.
I am thinking it must be a hardware fault from the onboard sound.
(board is p4s533-mx)