Hmmm. Sounds like a tricky one. Do you use custom sounds that you downloaded by an app like zedge or similar? If so, try to change to one of the preset sounds that are available by default and see if it works. I'm thinking it could be something wrong with the notification soundfiles. Have you installed any third party app that handles notifications/sounds? If so, try to run the phone in "safe mode" and see if notifications work.
...I assume you've already double checked so "Do not disturb mode" or the "Power saving mode" is enabled?
To start in safe mode;
1. Press and hold the Power button and Volume down button until the 'Power off' prompt appears on the screen then release.
2. Touch and hold Power off until the 'Safe mode' prompt appears then release.
3. To confirm, tap Safe mode .
The process may take up to 30 seconds to complete.
Upon reboot, 'Safe mode' appears in the lower-left of the unlock / Home screen. With Safe Mode enabled, ensure Airplane Mode is not enabled and test if you can receive notifications with sound.
