eggnoggpi/files/etc/systemd/system/mpv.service

[Unit]
Description=mpv audio service

[Service]
Restart=always
User=pi
Group=pi
EnvironmentFile=/etc/default/eggnogg

ExecStart=/usr/bin/mpv --loop=inf --no-audio-display /home/pi/music.opus 

[Install]
WantedBy=multi-user.target