I'm having an issue using my internal microphone. First time I noticed it was yesterday when I had a group call on Google Hangouts. After thorough research into this, I discovered that my problem was that one of my mic channels was phase inverted from the other, and thus the two channels were effectively cancelling each other out.
The mic would work if I used
sudo pavucontrol to unlock the channels and mute one of them out (and thus it would serve as a ground reference for the other channel), but the thing is that most apps (e.g.: Chrome, Skype) would auto-adjust the mic gain and re-lock the mic channels together, and thus the problem would persist. I tried everything I could to mitigate the issue, but nothing really worked. Also, I found out that this bug goes back as early as 6 years ago, and still no solution is available anywhere.