Sorry that no one responded to your query, but yes your issue sounds the same as the one where people complain about low volume on WhatsApp.
When on WhatsApp my daughter generally uses a bluetooth headset but when disconnected the volume is very low. Effectively the main microphone is not re-enabled ~ only the top noise cancelling one works ~ so talking closely and directly at mic 2 gives a hearable voice
I have never used a bluetooth headset and have no problem with WhatsApp ~ currently 
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