Hi,
i have a similar issue with my 930 updated to the latest W10M since one month.
Basically what happens is that my voice comes and goes on the other end, sometimes the other can hear me, sometimes not. Turing the speaker on, it works perfectly. But it's pretty weird that i have no issue during a call on Skype. My voice is always clean and perfect with or without the speaker on. So the issue is in the software not in the microphone, for some reason the noise reduction does not work properly during standard calls, or there are other issues.
I will try to restore to WP8.1 and see how it goes, then to jump to W10M and see if the problem persists. Maybe it is due to some features of W10M RS2 feature 2