I had a N9500GT in my othe HTPC and an Abit mobo... used the SPDIF cable and it ran just fine via HDMI but only after
I installed the nVidia HDMI audio driver. I don't game but it sounds like your game may need tweaking to passthrough
the 5.1 audio stream JUST like you'd do in a "player" for a DVD or TV show. Otherwise, this 785 mobo DOES have Dolby Home Theatre capability (page 92 in the manual
) which encodes 5.1 from Stereo on the fly. I'd think you really want the game's 5.1, not a Stereo signal adjusted to 5.1 though.
Read around... there may be issue on specific versions of nVidia's driver, maybe an older one works! Actually, maybe what you're using does -- you have got to prove the small parts: 1) are you connected correctly to SPDIF & GND; 2) is the cable connected to the 250 correctly; 3) do you have the nVidia HDMI Audio driver loaded, configured (as 2-speakers
) and passing anything
to your AVR; 4) what's the status of the HDCP (on the HDMI Audio device's settings) with the AVR (and display); 5) do you have DD, DTS as "supported formats" in the HDMI audio driver; 6) use a DVD or something with known DD/DTS audio you can play and verify the passthrough is working. THEN look into why your gaming is not giving you the desired audio.
One other thing you could try - as an education/proof - would be to remove the 250 and get all of these things working under the HD4200. Then you'd know how to configure drivers, configure the settings and get all of your functionality. Later you could add the 250 since then you'd know where to look to get all the settings the way you need them.