I have the same problem with my BV9500. But I don't think it's the microphone to blame. I used a call recording application on both BV9500 and another phone that works perfect (it's a BV6000 with Marshmallow on it) and called from bv9500 to bv6000, answered the call then I walked away and start talking on the bv9500; I can tell you this: on the BV9500 my outgoing voice recording sounds absolutely gorgeous. On the bv6000 side, my incoming recorded voice is awful. Also, sometimes, on bv9500 the signal drops from 4G/3G to G or E while being in a call. Anyway, regardless of the signal being G or 4G the outgoing voice is almost uninteligible. On the other hand, it's not a carrier system (telephony service provider) fault, because outgoing voice from bv6000 is very good. Maybe the tech team of Blackview should look on the radio firmware? And I want to reiterate the need of having the EngineerMode back (I know it's possible, the BV9500 sample provided to Shenzhen SEM Test for FCC had EngineerMode on it). It might help users to find solutions instead of complaining on the forum.
Ok, regarding the outgoing voice in a call: there is a workaround that worked for me. It's posted on 4pda and the following is a google translate from russian:
<<1. Find a patient and flexible interlocutor at the other end of the "wire"
2. Turn off the noise (the application "phone" - settings- accessibility- noise reduction)
3. In a calm tone, without hysterics and obscene words, speak with an agreeable comrade. Record the conversation to the recorder (During the conversation, the "Record" button appears. Adjust the volume of the interlocutor back and forth.
4. Listen to the recording. The interlocutor heard you in much the same way as you recorded yourself. If everything suits, close this topic.
5. Open the menu of secret settings
(sometimes it is called the engineering menu, dial in the "phone" mode * # 87 # (for the amateur version)
6. Click Single Test
7. Put the phone on the table so that it is about 30 cm from you to the phone. Press "Microphone 1". There will be a button on top that you need to press to start the test. Press the button, talk calmly, press again to finish recording the test. A button appears at the bottom to listen to the test recording.
They listened, clicked "Exit" (or something like that, I don’t remember exactly) [on bv9500 there is no Exit button here, just press PASS after you listen to the recording]
The setting is based on the assumption that the phone hears a signal weakened by the distance and automatically amplifies it. Exit from the secret settings menu.
8. Make pp 2, 3, 4.9. Turn on noise reduction (See p. 2)10. Enter the menu of secret settings (p. 5, 6)11. Press "Microphone 2" This is a noise-canceling microphone. Hold the phone near your ear, as when talking. Press the same buttons as in paragraph 7
. Easy to talk to. The phone will decide that you can hear it loudly enough, and reduce the sensitivity of the microphone (I think so)
12. Repeat pp 3, 4.>>