I cleaned out the tiny little microphone holes-- and it worked:
I was also having a problem with my internal microphone. In System Preferences > Sound > Input, I had the Internal Microphone selected, and the input level bar would show some activity when I tapped the top of the computer, but not when I spoke. So first I had to figure out WHERE the microphone actually was, and I googled and found it is on the top of my iMac-- a very small circle of TINY holes, right in the middle at the top of the monitor. Turned out I had covered it with a little decorative angel, using a putty usually used to stick things to surfaces. I removed the little angel and the putty... but was still not getting my voice to register in the input level bar in the Sound preferences. I used canned air to blow at the tiny holes, but that didn't help. Finally I found a pin (thinking it was actually too thick to poke the little microphone holes) but I WAS able to poke the little holes-- so I poked each of them-- and voilà-- my internal microphone now works! Yay!