Comment 3 for bug 1544477

On my MX4, then this is a permanent issue, that is, a reboot or forced-reboot (power button held for 10 seconds) does not clear it - opening Music and playing tracks, incoming calls and going into System Settings | Sound and selecting different sounds (Amsterdam, Positive, etc) all cause playing of the tune (with the stop icon at the bottom of the screen when in the System Settings | Sound screen).

It's just that when you leave the phone and an SMS comes in, it just vibrates and doesn't give any sound - it gives sound and vibration when a call comes in still.