alarm-clock-applet allows custom commands. So put systemctl suspend into a timer, bingo.

rtcwake to wake the computer up, for-better-or-worse the music will still be playing.

Someone else mentioned android, VLC there does have a sleep timer (just to stop the music) I didn't see an equivalent option in the desktop version (at a quick glance) though.

In SMPlayer I do see the option 'shut down computer' as a sub-option for `close when finished playback' (general options)