As asked by @Hui Wang, I have created a separate bug #1880571 for my problem on focal fossa and run apport-collect. As the data collected by apport showed that timidity was running I have uninstalled timidity and this seems to have fixed the problem: audio output was immediately restored, and after each of three reboots (the last of which after removing "options snd-hda-intel dmic_detect=0" from /etc/modprobe.d/alsa-base.conf), audio output was available right after logging in.
So my problem really seems to be a bug in timidity.
As asked by @Hui Wang, I have created a separate bug #1880571 for my problem on focal fossa and run apport-collect. As the data collected by apport showed that timidity was running I have uninstalled timidity and this seems to have fixed the problem: audio output was immediately restored, and after each of three reboots (the last of which after removing "options snd-hda-intel dmic_detect=0" from /etc/modprobe. d/alsa- base.conf) , audio output was available right after logging in.
So my problem really seems to be a bug in timidity.