We have what MAY be a similar problem. Namely for unexplained reasons the volume is low, or off, although we have done nothing to make the change from original volume settings.

 

Hi there,

May I know your software number (Settings > About > Software information > Software number) and the carrier you use?
Normally, once you set the in-call volume, it keeps to all calls until you change it again.
Do you use any profile setting apps?
You may try to make calls in safe mode to see if the in-call volume always reset to be low. If the issue disappears in safe mode, it means the issue is caused by 3rd party apps.
(Safe mode: Long press the power key and then long press "Power off" in the phone options. Press "RESTART/OK" in the pop-up message to get into safe mode. All 3-party apps will be temporarily stopped in safe mode. You may leave safe mode by a normal rebooting.)

 

Sadly this seems to be a hardware issue. When my one m8 was new you didn't even need the speakerphone to hear the incoming call if it was away from your ear. A couple of weeks ago I noticed a severe degradation in the in call volume. First I thought my hearing was going. Then I went through all the tests, settings, diagnostics, physical configurations and nothing really worked. I finally compared the in call volume on my m8 to my wife cheapy cell phone and the difference was so pronounced I decided it had to be a hardware issue.
After a whole lot of BS with Verizon and Asurion, their insurance for covering damaged or lost phones, they finally relented and sent me a replacement. But that's another story.
I am awaiting the replacement.