My HTC Desire 728 (Android 5.1) suddenly stopped emitting any notification sounds for Messages (SMS aka "text") and WhatsApp incoming messages.
First thing I did was to check the "global quick setting" for the speaker: It's on Normal (i.e. not silent or vibrate).
Then, I checked
Sound & notification >
Notification sound. Sound is selected as should be but when I click it (or press the volume up mechanical button), its volume is extremely low, practically silent. Repeatedly clicking it (or pressing the volume up button) increases volume until it sounds just fine. But as soon as stop clicking it, within 2-3 seconds it returns to the problematic silent volume setting.
This is not how it used to be and it appears that there is something (app? service? daemon?) that forces the volume back to silent.
I wiped the cache partition as well but that did not help.
Any idea how to solve this without needing to go through the hassle of entire factory reset?
Note: All other sounds (ring, media, in-call are fine). It's only the Messages/WhatsApp notification sound that misbehave.
UPDATE: Groping in the dark, I continued to troubleshoot and I found that the problem is in essence Notifications volume turning itself down. One can learn from that thread a factory reset does not help anyway. I will study that thread and report back.