After having followed this thread for a while I think we might conclude that:
1. the problem arises in several forms. The core problem being the mic not working for outgoinjg calls
2. the problem generally arises after some time. And it might be intermittent.
3. There is no fix. In a few instances starting from scratch seems to have helped (temporarely?)
4. Apple replaces the device the moment you metnion the mic-problem, it seems they are aware but have different priorities.
5. There are a few instances where replacements showed the same problem. This suggests that the problem does not concern a few in 100000 but rather a significant percentage, but only the replacement service really knows the magnitude of the problem.
5. To me it looks like an interaction of a hardware AND a software problem. This is possibly quite complex.
Every problem can be solved if enough resources are spent on it. Apparently Apple doesn't want to spend enough. I think we need someone with investigative journalistic talents finding out how big the problem is, what Apple is doing about it and subsequently informing a large journal.
Under pressure I suppose Apple with adjust its priorities.