The best reply for the microphone malfunction in Lumia 930 is found on page 1 and marked as answer for this long thread.
If you are sure that nothing is blocking any of the 4 small microphone holes in the Lumia 930, then the phone is mechanically defective and need repair.
Screen protectors and protective covers are known to cause microphone trouble.
Remove those and test again is the only troubleshooting step for this issue to try at home, before taking the phone to a qualified technician for repair.
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Hans