How to fix choppy/metallic HDMI audio issue:
Didn't notice this until last night and the fix that worked for me is pciPassthru1.msiEnabled = "FALSE" in this case HDMI audio is assigned as PCI device 1 so make sure you find the right number in your configuration.
I also tried one other tweak but that didn't help in my case but it may work for you. You can find the solution in post 323 in this thread by user derickso.
I also tried passing thru onboard HD audio. Win7 detects it but after that anything to do with audio playback comes to a grinding halt.