Please, help, my microphone is VERY quite!

Hi, everyone. I have Lenovo K900 Android version: 4.2.1. Recently I changed the battery since then my mic hasn't been working during phone calls, both regular calls and on WhatsApp. Although it works perfectly when shooting a video, making an audio recording, voice searching etc., only during calls. It's not completely broken if I shout right into the mic the receiver can hear me
very badly (as if I was talking underwater or from very far). Other things to point out:
1) I can hear people well, so there is no problem with my speakers.
2) Using speaker (in order to enhance the mic) during a call doesn't help at all.
3) It has a very problematic back cover, so it took a lot of physical effort to open it, so my suspicion is that mic could have been damaged, though it seems unlikely since it works very well outside the calls.
4) Using a headset mic during phone calls works perfectly.
5) I don't have a warranty.
Some things I've tried so far to fix the problem:
1) Hard reset the system (twice), both normal reset and hard reset (thus deleting all the apps, so I'm doubtful, it could be a software problem)
2) Turned off "Background Voice Cancellation" and put mic quality to "bright".
3) Turned off "Ok google" voice option.
4) Made sure that it wasn't my fingers covering the mic causing the problem
5) Rebooting the system multiple times.
6) Using headset during the call, ending the call with headset on and only then taking it off in hopes to reset built-in mic settings.
7) Downloading "Sound About" app.
None of these options helped, even slightly.
I really believe that it's fixable since the mic works everywhere apart from calls, and even there it works, just unbearably quiet. I have one last idea left, which is to change the Android version completely, maybe upgrade it. Please, if you have any ideas, respond, this bug is driving me crazy. Thanks, everyone, in advance for your help!

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