Exactly the same issue here.
Raspberry Pi 3 Model B
Raspbian distro (not Pi MusicBox)
Mopidy 2.1.0; installed from APT source, running as a service
gst-launch-1.0 audiotestsrc ! autoaudiosink WORKS
aplay /usr/share/sounds/alsa/Front_Center.wav WORKS
Still no audio from Mopidy.