Couple of followup questions for you HCE:

Have you tried changing what ports the sound uses in the BIOS?
Have you double checked that in the Midi configuration utility that yours is setup for 2 speaker? The other options are not supported properly under OSX.
I have the exact same chipset and ID and mine works fine. Make sure you remove any extra sound kexts ie. AppleAzaliaAudio, ALCinject, HDAaudio etc... There could be another kext conflicting with your AppleAC97Audio.
Make sure your hooked up into the green port on your motherboard(I know its the obvious but best to double check everything)