Just did some digging. The answer seems to be that your microphone may be physically blocked. Use a crevice tool from a vacuum cleaner for about 30 seconds to try clearing it. Other solutions have been compressed air (I would not recommend this) or shoving a toothpick/wire into the hole (again not recommended).
If you decide to try any of these, remember it's at your own risk. I don't like the idea of potentially shoving foreign bodies further into the works, but I kind of like the vacuum cleaner idea. It has worked for other people.
By the way, the microphone is located on the top of the iPad, just above the front camera.