The inital mythbuntu install didn't support any of this, stereo sound worked fine but no joy with the extra channels. alsamixer didn't display the Front/Center etc channels. This thread revealed the solution to the problem.
We need to explicitly add the intel hda module. Add the line:
options snd-hda-intel model=3stack-6ch
to file /etc/modprobe.d/alsa-base
than restart your PC.The new channels will probably be muted. Also at the far right of alsamixer (it scrolls) there is a "channels" setting, it needs to be changed from 2 to 6. then use
alsactrl store
to save the changes.Testing of the channels is easy with this call:
speaker-test -Dplug:surround51 -c 6 -t wav
This gets 6 channel support working with alsa, but we still need to activate it in Myth. Just got into MythFrontend|Utilites/Setup|Setup|General - Audio Page
Set the the Audio output device to
ALSA:surround51
The Mixer control may need to be set to "Master" or "PCM" - whichever works.
Note : This is all for analogue output, SPDIF is another kettle of fish ...