Independientemente de que sigáis por la configuración del XBMC, echa un vistazo a esto:

Número máximo de canales: 6

IMPORTANT - For HD audio to work then Max Number of Channels must be reported as 8 if anything other than 8 is reported then HD audio will not work even if DTS-HD and TrueHD are listed in the Encoded Formats box.
Windows Settings for AudioEngine - XBMC (en el step 5).

Hilo con el mismo problema y, curiosamente, receptor (Onkyo HT-R390, lo escribe mal en el primer post pero despues pone el nombre correcto):

You've misunderstood.

For HD audio to work then XBMC must see that the audio device can handle up to a maximum of 8 channels of audio, that does not mean all audio must have 8 channels just that the audio device must support that as a maximum, so 5.1 will work just fine.

If your device is only reporting 6 channels then you've probably got an EDID problem requiring an EDID override, this is a common issue for AMD GPU's.

To solve this you'll need to go to AVSForum for support, the 2 threads are:

EDID Overrides to solve bitstreaming issues for ATI 5xxx's
EDID Override Thread

Tulli is the expert on these matters over there.
Y la solución fue, efectivamente, un EDID override por lo visto:

I only want to say thanks to jjd-uk, tungmeister and all the people that read this. I solve my problem with EDID and, finally, I can play HDAudio content with XBMC

Thanks!!!! XDDDD
http://forum.xbmc.org/showthread.php?tid=170693

Estas cosas a veces se solucionan con una actualización de drivers, pero como la fecha del post es de agosto de este año me ha parecido interesante comentarlo no vaya a ser que el problema sea ese y os volváis locos con la configuración del audio sin encontrar solución.

Suerte.