Blocking the internal microphone hole
So the answer to this question involves 8th grade science. So brace yourself. The answer has to do with the change of medium for the sound waves to travel through. Solids can conduct sound much better than air in truth but the transition between the two states of matter requires a decent amount of energy to switch so by covering it you are trying to make it change not once but twice as it hits the outside and changes to move through the solid then hits the air and thus you expend much of the energy. This reduces the volume and can even negate it all together. (Ever heard of sound proofing? This is the concept of it they just find a substance that absorbs the energy rather than allowing it to pass) So now knowing this you may just want to remove the case or get an external mic. like the rest of us. Sorry for the lengthy answer.