Since the problem with the microphone annoyed me more and more (I talk every day several hours from the mobile phone), I have now put my hand to the device. Please keep in mind that this is almost certainly a loss of warranty ... but has anyone ever tried to claim a Chinese bill guarantee?
I watched a video at Youtube in which the BV9000 was disassembled. It is actually quite simple: 3 screws on the left, 3 screws on the right, 3 screws on the top and 4 screws on the bottom ... then carefully remove the cover (the back of the device reaches down to the display). Do not lift too far, otherwise you will tear off the cable for the finger scanner ... and exercise a little caution with the seal, which likes to hang. I have placed the device on the display and left the back cover slightly sloping on top. Then you see a black foam on the bottom right, under which (white poured) the microphone is. Carefully remove the black foam and gently scrape off the white silicone from the micro. At some point you can gently lift out the micro and around the micro there is a black silicone case. With this one must enlarge (possibly with a puncture pliers) the passage opening, because this small badass seems to fit so badly that it partly covers the opening of the microphone. Then place the Micro again with modified siliconefitting, fix with a little superglue, fold the foam again, screw everything and start after turning on the sound recorder ... for me, the pointer has immediately beaten much further ...
It may be that you lose the warranty and the phone may not quite be as deep sea suitable, as expected ... but you can now at least have a call! The case noise is still broadcast unduly loud, but the phone has improved worlds!

Sorry for my bad english, this is a google translation to a post I made in a german forum:
https://www.android-hilfe.de/forum/blackview-bv9000-bv9000-pro.3233/mikrofonproblem-hardwareseitig-fast-geloest.892086.html

Thanks for your precious feedback!

Just a confirmation on this approach.

I attempted this repair today, and as pointed out above the hole in the rubber grommet did not line up with the mic hole.

I simply cut a larger hole, reinstalled (using super glue to seal around the mic - although I'm sure silicone would be a better option) the mic and reassembled the phone.

Useful to note - the screws on the side have smaller heads than those on the back. You'll work this out if you put the back screws in the side - the heads will protrude.

Highly, highly likely this is the cause of the 'muffled' microphone that a lot of people are experiencing.

Very easy fix, but likely voids your warranty.

i made this thread more visibile