I don’t know the reason why audio is off-center yet, so identifying that would be the first step 
The gain isn’t lowered that much though, and it only applies to L5 microphone - so audio that the other party hears. If you find the other party too quiet, just adjust the volume with volume buttons on the side or volume slider in phosh quick settings.
There’s module-echo-cancel in PulseAudio and there’s GSM codec in the modem. For the former, you can test how it works even on your PC, since it’s a regular mainline module included by default in PulseAudio installations. When it comes to the latter, since there’s no VoLTE enabled by default, the modem uses 3G codecs (I know that VoLTE can provide higher fidelity audio than that, but I don’t know the details).