There is a flaw in the conclusion blaming this error on software or firmware. You forget the people who have stable microphone performance on their fully updated Lumia 930 phones.
It's a problem in all vendor forums, mostly users with problems come here (and some who think sharing and learning is a great pastime, of course!)
I do not understand why a warranty claim were not made ASAP when the error occurred. A stable error in a basic phone feature is easy to demonstrate at the place of purchase - which is where defect claims are made within the E.U.
It is in the best interest of both the manufacturer and the retailer to maintain happy users. Some commitment is required, and a polite approach helps.
Trust me, I'm old. ;-)