I had that problem for a bit as well. I fixed it by taking the case back off and using my finger to gently press down on the gasket around where the microphone is. Just press gently. You dont need to feel anything move or click. Just give it a couple presses, then when you put the phone back in put it in bottom first and rotate the top in and make sure the case is pressed tightly together. That fixed the muffled sound issue for me.
IF you notice buzzing sounds coming from the speaker, take the case off and put a small piece of folded paper between the back of the phone and the case. This will stop the buzzing. It does not need to stay there forever, just until it helps keep the case slightly pushed out from the back. The buzzing comes from the back vibrating and rattling against the back of the phone.