I have the same MB and have working audio under Mountain Lion.
My setup may be slightly different than yours, as I am running the UEFI BIOS (U1H) and therefore am working without a DSDT.
To enable audio for my 889 Audio chipset, I run Multibeast and select ONLY these two:
Drivers & Bootloaders -> Kexts and Enablers -> Audio -> Realtek ALC8xx -> Non-DSDT HDA Enabler -> √ ALC889
Drivers & Bootloaders -> Kexts and Enablers -> Audio -> Realtek ALC8xx -> Patched Apple HDA -> √ ALC889
Make sure that you don't have a lot of legacy install audio kexts cluttering up your System/Library/Extensions folder, too. If you've got other Multibeast-installed audio kexts in there, remove them using KextWizard, rebuild permissions and reboot.
Hopefully this helps.