Re: No audio from HDMI (MCP89 / Geforce 320M): Reason: no connected devices detected
I'm not at all familiar with audio issues, but I'll try to help...
On the problematic card, can you reproduce the problem, run nvidia-settings, go to the "X Server Display Configuration" page, and click "Detect Displays". Does this show all the connected displays correctly?
There is also a page for each active display device near the bottom of the list. On this page is an "Acquire EDID" button. Can you save the EDIDs of all your monitor(s) and attach them here; then we can double-check that the EDID contains the correct audio data.
You mentioned that you have two GPUs and one works, one doesn't. Does the problematic behaviour follow the GPU or the display device? (In other words, try swapping the monitors, is the MCP89 always problematic, or is just one of your monitors problematic).