What happened is when you put the adhesive strip down you plugged the hole in the case covering the microphone.
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Here’s more details on properly installing the display [https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+Retina+4K+Display+(2017)+Adhesive+Strips+Replacement/96196]
The microphone is well hidden!
[image|2038911]
What happened is when you put the adhesive strip down you plugged the hole in the case covering the microphone.