If the mic works for all other calls and all other things on the device, then the problem isn't the microphone or the phone. The problem is with the app. You would have to contact them about the issue.
I was facing the same problem. But i tried it. Go to google app>setting>voice. Then toggle off 'block offensive language'. You can also add different language at hoogle app settings. This solved my problem. Hope it will solve yours too.
I had a same problem with 6.1 plus. I checked with Nokia customer support through the app. I got a reply saying to test the microphone by dialing *#*#372733#*#* after dialing the code I tested primary microphone by making some voice attempts but I got a long beep voice from the speaker, secondly I tested microphone 2 it is working fine.