Sound on iPad is very muffled when on a FaceTime call

Have you tested the microphones with other conferencing Apps - or the iPad’s Voice Memos App?

If audio is not clear, you would be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple

Had same problem; all troubleshooting steps didn’t work. The very last thing we tried was to check for obstructions on and around the microphone next the camera. Low and behold this was the problem. The screen protector did not have the proper cutouts or holes in the correct position. Make sure that the microphone isn’t covered. Once removed everything worked perfectly.

Have you placed the iPad in a fitted case? Some third party cases may obstruct one or more of the iPad’s microphones.

If the iPad is in a case, remove it - and then test FaceTime.