Hello anarchoi, I'd like to chime in on the exact same issue, different setup.
I currently have a XFX 7970 with four displays attached, two DVI monitors, one TV through either the HDMI or my miniDP to HDMI adapter, and also an Onkyo receiver on again either the HDMI or miniDP with HDMI adapter.
I say either because similarly, I have been swapping the cables to get audio where I want it. The TV is in a bedroom in the other room, the Receiver is in my main home theater room (the DVI monitors are at the console/tower). I generally use DisplayFusion to swap display to where I need it, and it works perfectly fine with video, but the audio is stuck.
I've noticed that the card will designate an HDMI audio device on either the HDMI jack or miniDP to HDMI adapter depending on which is plugged in alone during windows boot up. Once it's detected, cables must be removed, and restart to switch them. Since it can work on both signals, I assume this is a fault with the driver itself. I am currently running the 13.2 beta drivers (due to the artifacting issues with 13.1)
Also, in a semi unrelated note, when I use my miniDP to HDMI adapter on a 50ft HDMI cord, I get the white snow in black images, not so with the actual HDMI port. I wonder if my adapter is at fault, or the display port spec has less power and can only tollerate shorter cable lengths. (50ft is pretty extreme in most cases regardless)
Anyway, I too home AMD reads these boards and can help us out here.