I am having the same issue on the same board. The version of PC EFI i am using makes SL install the 64-bit Kernel by default. My understanding is that all of the kexts and fixes on the forums only work in 32 bit mode. I am having trouble figuring out how to install a 32-bit Snow Leopard to test the theory.

If you click on "Software" in the System Profiler, you should be able to see if your Kernel is 64-bit as well.

In the meantime, I have a Behringer USB audio interface that gives me sound output, but I would rather just use that for recording.