I noticed you mentioned using nVidia as your GPU.
I was having similar issues with my PC, and when everything was said and done, the HD Audio driver for nVidia was causing my PC to freeze while using audio through HDMI using the HD Audio driver.
My freezing began when I purchased a plasma television to be used as a monitor for gaming; I normally use USB headphones for audio. After using the HDMI connection from the GPU to television as the audio medium, my computer would start freezing randomly while audio was being used through the television speakers.
After uninstalling the nVidia HD Audio drivers and using the optical port on my motherboard to the television, I experienced no other problems.
Maybe this will help you out.