Note on the newer iMacs, the microphone is located on the right side of the lower edge. Make sure not to cover this up with tape or your microphone won’t work.
I know this is an old question, but I wanted to post a followup so hopefully, no one else makes this mistake. The microphone is located on the lower edge. Here’s the thread that covers this better. Microphone very muffled after sealing up iMac