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There is also a thread like this in Z2,perhaps something helps:
/t5/Xperia-Z2/Issue-with-microphone-when-using-speakerphone-in-5-1-1/m-p/1019564#M31289
wait, there is already a 5.1 update for the Z3+? 
No,I suppose it is Z3. Moved it to correct section.
I'm also having the same Issue and realize that the real problem relies on Google app. If you disable the "OK Google" recognition on any screen the mic will work as it is..
No, it does not.
I was using ok google detection and google launcher, gave up after this bug. Nope, nothing has changed; same microphone problem is there.