This was a pain to find and install. It doesn't seem to be supported by Microsoft (though some people report seeing the driver in their distribution). I'm running Vista Business on an (old) Sun W1100z. Basically I managed to install it by getting hold of the Windows® 2000/ME/XP AC’97 Audio Driver Version 1.8.7 or getting the X_1_8_7.zip package (and unzipping it) then going to Device Manager and right-clicking the duff audio device entry and select 'Update driver software...' - choose 'Browse my computer for driver software' and selecting the location where you unzipped the driver package to - making sure that search sub directories is clicked. It should find the driver there.
AMD also have a driver pack that contains all the drivers for the whole AMD-8000™ Series Core Logic - which provides for a variety of functions (USB, HPET, AC'97, etc) - rather like Intel's 'SouthBridge'.