When I tried playing a movie in XBMC, however, all I could get was stereo. I went into the System > System > Audio settings and set Output=HDMI and enabled both 'Dolby Digital capable' and 'DTS capable'. But then the movie went silent and stuttered badly. I tried the same test with Media Player Classic - Homecinema and got the same result. What did I have to do to get surround-sound?!
After much research I finally determined the following:
- The ATI 785G chipset supports DD and DTS passthrough on HDMI but not with the standard ATI drivers! (It does not ever, however, support HD audio-passthrough, e.g. from Bluray)
- The standard ATI drivers create both an "ATI HDMI" audio device and a virtual Realtek SPDIF device. Deleting the latter doesn't help, nor is it recreated.
- The audio chip on the motherboard is from Realtek and they provide their own alternate (better!) driver, www.realtek.com.tw/downloads
- You can download the improved driver at their web-site under High Definition > ATI HDMI Audio Device
UPDATE 9/14/11 - I started having problems where the audio-link wouldn't sync-up (i.e., no sound) even though the Windows volume-control said it was working. So I uninstalled the Realtek HDMI driver, then updated to the latest ATI Catalyst 11.08 driver. Everything seems to be working now, and without the 'unofficial' fix!