PulseAudio

When running Mopidy as a service, it runs as the mopidy system user rather than your own user. PulseAudio runs as your main user, so Mopidy can't access it directly. Running PulseAudio as a system-wide daemon is discouraged by upstream. Instead, you can configure PulseAudio and Mopidy so that Mopidy sends audio over TCP to the PulseAudio server already running as your main user.

First, configure PulseAudio to accept sound over TCP from localhost by uncommenting or adding the TCP module to /etc/pulse/default.pa or $XDG_CONFIG_HOME/pulse/default.pa (typically ~/.config/pulse/default.pa):

### Network access (may be configured with paprefs, so leave this commented
### here if you plan to use paprefs)
#load-module module-esound-protocol-tcp
load-module module-native-protocol-tcp auth-ip-acl=127.0.0.1
#load-module module-zeroconf-publish

Next, configure Mopidy to use this PulseAudio server:

[audio]
output = pulsesink server=127.0.0.1

After this, restart both PulseAudio and Mopidy:

$ pulseaudio --kill
$ start-pulseaudio-x11
$ sudo systemctl restart mopidy

If you are not running any X server, run pulseaudio --start instead of start-pulseaudio-x11.

If you don't want to hard code the output in your Mopidy config, you can instead of adding any config to Mopidy add this to ~mopidy/.pulse/client.conf:

default-server=127.0.0.1