Muffled Microphone - iMac 21.5" (mid 2017) - Following Adhesive Strips
This is actually an answer rather than a question; a solution to a problem that is not yet in any guides. Since I couldn't find a resolution online, I figured I'd share my own.
The mid 2017 21.5" model of the iMac is new and this issue isn't major, so it's not surprising that the few people who have sewn up their computers after an upgrade or fix either haven't encountered it yet, or found the true cause.
The problem is that after reassembly, the microphone input is faint and muffled. It functions, but barely, and is essentially unusable. The culprit turns out to be the adhesive strips used for reassembly. The frame of this iMac model seems identical to its 2012-2015 predecessors, so ostensibly the same strips made for these earlier models would work here too. However, the microphone has been moved from the top of the computer (near the webcam) to behind the frame at the bottom of the screen.
Note the location of the "4R" strip in the following picture:
Now look at the location of where the microphone has been moved to:
One can clearly see that the "4R" strip completely covers the microphone and the holes in front of it, deadening any sound picked up.
Knowing this, the solution is simple. Make sure the microphone works properly before using any adhesive. Then, apply the adhesive strips, but when laying down the "4R" strip, cut away the portion covering the microphone (and a little bit more from each side to be safe).
Hopefully, this post will save others from hours of exasperation! :)