I have an Intel ALC892 and a Nvidia GT 520m connected to speakers via HDMI.
On lspci, I see
00:1b.0 Audio device: Intel Corporation N10/ICH 7 Family High Definition Audio Controller (rev 02) Subsystem: ZOTAC International (MCO) Ltd. Device a218 Flags: bus master, fast devsel, latency 0, IRQ 47 Memory at db400000 (64-bit, non-prefetchable) [size=16K] Capabilities:  Power Management version 2 Capabilities:  MSI: Enable+ Count=1/1 Maskable- 64bit+ Capabilities:  Express Root Complex Integrated Endpoint, MSI 00 Capabilities:  Virtual Channel 02:00.1 Audio device: NVIDIA Corporation HDMI Audio stub (rev a1) Subsystem: ZOTAC International (MCO) Ltd. Device 2180 Flags: bus master, fast devsel, latency 0, IRQ 18 Memory at db080000 (32-bit, non-prefetchable) [size=16K] Capabilities:  Power Management version 3 Capabilities:  MSI: Enable- Count=1/1 Maskable- 64bit+ Capabilities:  Express Endpoint, MSI 00 Kernel driver in use: snd_hda_intel
My alsamixer looks like
I enabled pulseaudio configuration file to have 6 channels. My sound setting looks like
When I use the test dialog, only front left and right have sounds. If I use alsa in XBMC on a 5.1 video, there's no sound. If I use pulseaudio, only front right and left have sound. I can barely hear any speech since I'm guessing it's mapped to front center. Any clues?
Update: on what I'm hearing on Ubuntu's IRC, I can't do 5.1 over HDMI and my best bet is to encode AC3 live over S/PDIF using A52 but I can't compile alsa-plugins... it gives an error somewhere while building oss. Any second opinions?