Go into device manager and make sure the drivers for your card are ok. I remember when I put a GT 430 in my friend's PC and installed the drivers and everything and the HDMI audio still didn't play, I just happened to check device manager and there were yellow question marks on the devices that were disabled for some reason. So i updated them and all of the sudden the digital audio option popped up in control panel and all was well in the world.
Still having this issue. I've tried a c ouple otehr things to no avail. I'm about to reformat to see if that helps. I can get audio out of the front port (and have used my game c hairs wireless audio, which has been nice), but still want it to come through TV for movies and stuff.
Before I do, I wanted to see if anyone had any other suggestions.