Sorry that you’re having issues, here are the steps I would take:
# Use a voice recording app to really check if the microphone is either blocked or not working properly.
# Check the small holes at the bottom of the device next to the charge port for any pocket lint or dust.
# If you know for sure it’s a constant issue and there is no dust/pocket lint, you’ll most likely need to open the device.
# After disassembly which can be found here: [guide|104605], you’ll need to replace the daughter board, because the microphone sits on that board. The part where he starts working with the daughter board starts at step 13.
# The part you’re interested in is at 3:02 of this video: https://www.youtube.com/watch?v=UpjG0s057LA, he labels it as ‘sub board’, and I cant seem to find a sub board sold in the US yet.
# Places I can find it are: https://www.ebay.com/i/223273564334?chn=ps' . and . https://www.nutelsolutions.co.uk/official-huawei-p20-pro-antenna-sub-board-02351wsw.html .