What I'm saying is that there are two more or less completely unrelated hardware blocks. There's usually an audio controller in the chipset along with the audio controller in the GPU. The one in the chipset is connected only to regular jacks on the motherboard; the one in the GPU is connected only to the HDMI output block and GPU hardware combines the video and audio info into a single stream.
The issue AFAIK is that the audio driver either doesn't support or isn't enabling multichannel output. No graphics knowledge is required to work on the audio driver. You do need some GPU knowledge to work on the combiner block in the GPU - Alex has already said that the GPU info we released recently should be enough to enable *combining* multichannel audio with video on the HDMI output if the audio controller is *generating* a multichannel audio bitstream - but Rafel and others already have that knowledge.