You’re onto something here. I’m pretty sure this isn’t a mopidy config issue. I did as you instructed and it didn’t help, BUT I went further and ran these tests:
aplay /usr/share/sounds/alsa/Front_Center.wav
It played thru the hifiberry as expected.
sudo -u mopidy aplay /usr/share/sounds/alsa/Front_Center.wav
It played thru the HDMI (unexpected).
This made me think there was a user mopidy vs user pi issue, so I tried this:
sudo -u pi aplay /usr/share/sounds/alsa/Front_Center.wav
This played thru the HDMI. (UNEXPECTED!) and this makes me believe my issue is with Alsa setup. Somehow…