Another experiment with USB audio
Trying to work out how to automatically switch to USB-C when I plug in the headphones (rather than switching manually as previously discussed, by typing on the command line).
pacmd load-module module-udev-detect
works, partly: that is, it will detect the headphones when plugged in, and shows “Usb speakers” when turning the volume up or down. BUT playing music using the Music app doesn't work, i.e. no sound comes out. So manual switching is still necessary when using the Music app.