Hi have you done a decent search of this forum, I’m sure there are quite a few posts on the issue, and have you asked Fairphone ~ that would seem to be the better option.
On the FP3 the top mic is used for hands free. I would imagine something similar for the FP4 but will check.
Meanwhile search the forum
See also
https://forum.fairphone.com/t/microphone-quality-of-the-fp4-when-using-signal-low-volume/79021/9