Andoid Nexus 5X Microphone sound too low
Your problem root is the "Noise Cancelling" feature.

These are the possible replies for your question :


Disable Noise cancelling feature [ROOT ONLY]. 
Put a little piece of tape on your Noise Canceling Microphone.
Use Headset or Bluetooth Microphone.
Wait for an OTA Update (Hopfully it will resolve your issue).