SOLVED:
Additional info on how to add/configure the mopidy AddOn:
- TTS using built in RPi3 analogue audio output with Hassio? (thanks @MacGuy76 !)
- https://www.amalytix.com/en/blog/home-assistant-with-conbee-ii-deconz-xiaomi-aqara-and-hass-io-from-scratch/#play-audio-files-mp3-wav-via-raspberry-pi search for " Play audio files (MP3 / WAV) via Raspberry Pi" on that page (thanks @trutz !)
In my setup the share directory was expected to be under /usr/share/hassio/
(All those different deployment models are quite confusing for a simple newbie like myself)
sudo mkdir -p /usr/share/hassio/share/mopidy/playlists
sudo mkdir -p /usr/share/hassio/share/mopidy/media
chrisV