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Check to see if you have Microphone noise suppression activated
Welcome to the club! It's my understanding that this is a product of the water proofing, and occasionally it bugs the crap out of me. Sadly I don't have an answer for you, sometimes it's OK, other times it's poor.
I have the same problem (sometimes the quality is OK, most of the time ppl tell thay can bareely understand me and I sound as if was under water) and the funny thing is I am using a handset provided by Sony as a replacement for my previous phone which was leaking water. I tried with the noise suppresion off (no luck) then I sent it to Sony and got it back with a note that it was fine. I called Sony customer service from the phone and they admitted the quality was awful and booked me a "VIP" repair service. Shortly after I received the phone back with a note saying it was fine. Happy days. I just got a Moto G 2nd gen as a temporary/back up phone and I will not buy Sony products anymore (so disappointed as overall I loved the Z2).