Hi there,
I did a test with Lumia Phone Test Application.
Microphone 2 does not work fine and that is the issue. It is not broken, it's active and it catches sounds, but the volume is super low. So when I speak to it, it's like if I am miles away. And probably the mic 2 is the one used for noise reduction during
the calls, or it has similar functions.
Since it works bad, the phone may think that nobody is speaking, or there is too much noise. Don't know how it interprets the data it receives from that mic, but at least I found out the culprit.
Mic 2 should be detachable from the motherboard, it seems it is not welded to the rest of the phone like other microphones are, so I will try to change it by myself. On eBay the mic i need costs just some euros.