to see what might be wrong, its reasonable to first look into the service’s status with
service mopidy status. There you can see whether the service is up and running and also see the latest lines of the log.
To get more log insights you can also go to your system’s log and filter it for lines including
cat /var/log/syslog | grep mopidy.
If there is a problem with pulseaudio, I’d expect to find an error message like
ERROR [MainThread] mopidy.audio.gst GStreamer error: pa_stream_writable_size() failed: Connection terminated or similar.
One thing you can do is looking whether the mopidy user can play anything using the pulse server you specified with
sudo -u mopidy paplay -s 127.0.0.1 [Path to some sound file] . You could use
/usr/share/sounds/alsa/Noise.wav as test file if you don’t happen to have something else on your disk.
If the paplay fails, mopidy is innocent and the issue lies somewhere in pulseaudio.
Hope that helps for a start