This is the new position of the microphone of the iMac Intel 21,5 inch Retina 4K Display 2019 and apparently also in the iMac 21,5 inch of mid 2017...

The adhesive strips set that is advised to use for this iMac, covers the microphone hole. That is the reason of the muffled/no sound after assembly.

I learned this the hard way and need to disassemble the iMac again and order a new set of adhesive strips but I hope I prevent you from making the same mistake.

The solution is simple. Just cut the right lower adhesive strip at the place where the Microphone is now situated and your good to go!