As you probably discovered you can see the groups a user is in by running groups <username>.
As to why there is no sound, first stop is a simple config (i.e. NO FIFO) and:
https://docs.mopidy.com/en/latest/installation/raspberrypi/#testing-sound-output
then try a gst-launch example from:
https://docs.mopidy.com/en/latest/audio/
then ensure your soundcard is as expected with aplay -l etc