To sum things up... Here is a statement issued by Windows Audio Quality team member on reddit.com/r/windowsphone
"Hi, Windows Audio Quality guy here. Here's what we think we know about the microphone issues on the Lumia 930.
1. The Lumia 930 uses several microphone elements that work together to deliver sound to the OS.
2. Due to known hardware issues, sometimes these microphone elements go bad and stop producing sound. When this happens, they don't ever fix themselves. So you are now in a situation where only some elements are producing sound, and others are not.
3. Different apps record audio in different ways. Depending on which element(s) went bad, and how the apps record audio, things might seem to work anyway, or you might get silence.
4. The built-in Camera app changed the way it recorded audio between Windows 8.1 and Windows 10. So some users might notice that going from Windows 8.1 to Windows 10 breaks recording video... and other users might notice that going from Windows 8.1 to Windows 10 fixes recording video! It all depends on which elements went bad.
5. The only way to fix the problem, that we know of, is to replace the phone."