The HDMI monitor that I am using is not listed in audio settings. This is a known issue (27.1 Release Notes, Implementation 2.2). The workaround stated is:
$ pulseaudio --kill
$ systemctl --user enable pulseaudio.service
Unfortunately this had no effect on my system. HDMI sound on this board/monitor worked previously on 27.0.1. Are there any other workarounds to try?