As usual, Google was both my friend and my enemy -- it found lots of results for all of my search queries, most of which sent me off chasing wild geese. For some reason, Application > Settings > Settings Manager in Mythbuntu does not provide anything to configure sound, and installing the unity-control-center package did not remedy that. (I wonder if they block it to prevent users from configuring something that conflicts with the MythTV audio settings, which you set when configuring the MythTV front end?) I ran alsamixer and unmuted S/PDIF, but that did not fix things.
I did notice at some point that the configuration for ALSA (the sound system) at the operating system level seemed to default to PulseAudio. I read somewhere that MythTV, which uses ALSA, might kill the PulseAudio service to avoid conflicts with ALSA. That suggested either creating a configuration file for ALSA that avoids PulseAudio or configuring PulseAudio properly. I tried the latter and it worked. The steps were:
- Install the pavucontrol package from a repository.
- Run pavucontrol in a terminal.
- In the Configuration menu, choose the correct output device (in my case, "HDMI").
- Exit, cross fingers, reboot and test.