So, here is the final solution for Lumia 930 microphones test !
The tool described here allows to perform the test of 930's microphones independent from the operating system installed (it switches the device to the "test mode" which seems to be some kind of "BIOS" or firmware related tool).
So, if any of the microphones fails the test, skip any hard resets or OS flashing, just take your phone to the service point.
Mine had been repaired (2 microphones exchanged last October) and now it seems two remaining ones need to be replaced. At least, one of them is completely silent while second one is recording "something" but not with the quality equal to microphones 1&2.
The tool described here allows to perform the test of 930's microphones independent from the operating system installed (it switches the device to the "test mode" which seems to be some kind of "BIOS" or firmware related tool).
So, if any of the microphones fails the test, skip any hard resets or OS flashing, just take your phone to the service point.
Mine had been repaired (2 microphones exchanged last October) and now it seems two remaining ones need to be replaced. At least, one of them is completely silent while second one is recording "something" but not with the quality equal to microphones 1&2.