[mythtvnz] No sound on videos

[Stephen Worthington]

On my mother's MythTV box, I have just tried to play a video file (as
opposed to a recorded TV program) for the first time, and there is no
sound.  I tried using the builtin player, and using mplayer.  The
"Audio output device" setting is "ALSA:hdmi" - the sound goes via the
HDMI audio to the TV, and the TV sends it to the amplifier.  It works
fine for TV programs.

I have tried playing a DivX/MP3 AVI file, and a flash file (VP6/MP3).
Both files play with sound on my own MythTV box using the builtin
player.  My box has the "Audio output device" set to "ALSA:default"
and the sound goes via SPDIF to my DAC and amplifier.

Does anyone have any ideas what I should do to fix this?  Do I need to
set my mother's box to "ALSA:default" too and find some way in the
ALSA config files to send the audio to the HDMI port?  I have never
found any good documentation on what to put in the ALSA config files -
they seem to be a black magical art.