Check your Mopidy logs to see if there any errors reported.
You could try an internet radio stream or a local file to see if your problem is only related to Spotify audio or Mopidy audio in general.
You can also try https://docs.mopidy.com/en/latest/installation/raspberrypi/#testing-sound-output
and also run:
gst-launch-1.0 audiotestsrc ! audioresample ! autoaudiosink