Comment 5 for bug 1544477

The volume for notifications is currently separate from the ringtone (default) setting. I suspect yours is at the lowest setting. Try turning up the volume as soon as an incoming notification is on screen. And yes this is rather tricky to do and we will modify this behavior in the future

Alternately from the command line you can remove all the current settings and go back to defaults:
rm /home/phablet/.config/pulse/ubuntu-phablet-stream-volumes.tdb
and restart the phone.