Dear ttttttaaas,
Bad news.
I have done a full restore of my iPhone 7 Plus, not using any back-ups, just the iOS11 only. Even turned the phone off and on after activation.
The mic-cutting-out problem is still there.
Actually my senior AppleCare guy was super interested yesterday and today to find out, and in the afternoon he reached out to me if I installed the iOS11 already. If so he wanted to test it with me to find out if the problem was solved. This evening (The Netherlands) we were able to do as such and after the test between him and me, I could hear in his voice that he was seriously disappointed when he had to conclude that the problem was still there.
He happened to receive half an hour before from Apple Engineering an email, to ask if the iOS11 had fixed the mic-cutting-out problem that we have been addressing to them for quite a long time now. My senior AppleCare guy will inform that it's not the case, that the problem is still the same. So Apple Engineering is already informed that there's no improvement to this issue with iOS11.
We are all quite disappointed.
My AppleCare guy will keep me in the loop regarding next steps or when new information is received from engineering.
May I encourage you and others to keep reaching out to AppleCare by phone. And address this problem.
I am seriously disappointed. Was really hoping for a solution by now. Many many months I've spent on this problem with my senior AppleCare guy. I should not loose faith regarding a fix for this issue, but my 'faith' is quite dented at the moment.
Please share your experiences here and inform AppleCare via phone directly as well, so they receive as much feedback as possible. That's most important. As much direct feedback as possible. Not only this thread, but real conversations with AppleCare people where they can and will log and register all the various users with this same problem.
When I know more, I will post it again here.
Take care for now.