Microphone not recording audio

Whenever I repair an iPad (or any device) I always check the microphones, speakers and headphone jack by recording a video with both the front and back camera. I play it back and listen via headphones and speakers. If it's a phone, I make a test call if possible (SIM card). Some devices have three microphones.

In your case, it's not clear from your question if the problem is lack of recording or lack of playback (or both). Perhaps you can clarify by commenting on this answer.

If you can record audio but it wont play back on the headphone jack, inspect the headphone jack for debris or jammed contact pins inside.

If you can't record audio but the speakers and headphones work, then normally a new microsphone flex would solve it. That said, small parts are notoriously hit or miss. Did you buy it fro a reputable vendor (like iFixit) or via eBay or Amazon?

Otherwise, go bak and check all your steps. Inspect the flexes for tears or damage. You may have a logic board problem, as Hortman says.