Microphone won't work after Fusion Drive replacement

Often times the small gap and channel between the aluminum shell and the display glass to the microphone gets blocked so the sound can't get to the mic. If you open the audio control panel it offers a volume level display with the internal mic selected if you hit the glass bottom middle edge does the mark move? If it does that a good sign the opening is clogged. This could be from your adhesive removal process when cleaning the surface, or the tapes you used did not offer the needed gap so they cover the mic hole. Reference Step 14 in this guide iMac Intel 27" Retina 5K Display 2019 Adhesive Strips Replacement.

If you don't see any response from the level meter, make sure the mic cable is not damaged and the ZIF connector on the logic board is correctly set. Reference Step 53 in this guide iMac Intel 27" Retina 5K Display 2019 Blade SSD Replacement and lastly also check the cable from the headphone jack in Step 52. You might also want to see if a headphone with mic works as well as that is also a useful diagnostic step if it fails to either record or allow you to hear then the jack & cable Is bad (damaged).