Why does my iPad sound muffled during phone calls?

Your iPad has multiple microphones used for different purposes, including noise reduction and echo cancellation.

Are you using a screen protector or case? Many models of iPad have a pinhole microphone aperture adjacent to the front facing camera - this being in the centre of the long-edge of the screen bezel. Some screen protectors or cases can cover the pinhole microphone.

Similarly, unless properly designed for your iPad, some third-party fitted cases can cover your iPad's microphones.

If you are using a Screen Protector or fitted case, remove both from your iPad and re-test.

Are you using a fitted case? A poorly fitting case may also obscure one or more of the iPads microphones.

If you are concerned that your iPad or its microphones are not working properly, you would be best 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. If a fault is identified, you will be in the best possible place to explore your a available options:

Genius Bar Reservation and Apple Support Options - Apple