- In the phone dialer type in *#*#197328640#*#*
- A menu should now be shown. In the menu, press  AUDIO
- …then  NB (VOICE CALL)
- …then  HANDSET
- …then  VOLUME
- …then SRC SPEECH RX Volume
- …then 5_lvl: 88 (note that you might have another value than 88 on your handset. Write down this value in case you want to revert to it later)
- The window that is now shown should have a line saying “Input?”. To input a new value press the menu button, then “Key Input”. Write 110 as the new value and press “OK“.
- You have now adjusted the microphone volume when noise reduction is disabled, and it is time to do the same when noise reduction is turned on.
- Press the menu button, and then “Back”. Do this four times in total, until you get back to the menu that the text “NB (Voice Call)” in the first row.
- Now, press  HANDSET(2MIC) and repeat steps 5 to 8 listed above.
I have had no complaints about the microphone volume after I completed these instructions. Hopefully it applies to you as well! The only downside with this trick is that it won’t survive a restart of the phone
Disclaimer: Although it is highly unlikely that it should happen, I take no responsibility if these steps should brick your device.