To create a respawn service with systemd just create a file in /etc/systemd/system/ i.e. mplayer2.service

[Unit]
Desription=mplayer with systemd, respawn
After=network.target

[Service]
ExecStart=/usr/bin/mplayer -nolirc -ao alsa -vo null -really-quiet http://stream.sunshine-live.de[..]cebook-og-audio-tag/
Restart=always

[Install]
WantedBy=multi-user.target
and activate it

systemctl enable mplayer2.service
reboot or start it manually

systemctl daemon-reload
systemctl start mplayer2.service
If you reboot or kill the process, it will be restarted automatically some seconds later.