PipeWire

When running Mopidy as a service, it runs as the mopidy system user rather than your own user. PipeWire runs as your main user, so Mopidy can't access it directly. As with PulseAudio, you can configure PipeWire and Mopidy so that Mopidy sends audio over TCP to the PipeWire server already running as your main user.

First make sure that pipewire-pulse is installed. It's PipeWire's PulseAudio replacement.

Check whether a configuration file for pipewire-pulse is available (may depend on the Linux distribution but /etc/pipewire/pipewire-pulse.conf is standard). If not, copy from /usr/share/pipewire/pipewire-pulse.conf.

Then modify that file for pipewire-pulse to accept sound over TCP from localhost (note the uncommented line with "tcp:4713"):

pulse.properties = {
    # the addresses this server listens on
    server.address = [
        "unix:native"
        "tcp:4713"
    ]

Next, configure Mopidy to use this pipewire-pulse server:

[audio]
output = pulsesink server=127.0.0.1

After this, restart both PipeWire and Mopidy:

$ systemctl --user restart pipewire pipewire-pulse
$ sudo systemctl restart mopidy