I have have had a conversation with a "level 2" support person for 10 days, all was going into the right direction; he admitted that iOS12 does a "stress test" on the hardware to see if it is fit for future purpose and that this stress test "can occasionally break the microphone module" but that it was due to fail anyway if that happened.
In my case it was not only the mic playing up though.. I have had multiple issues with the health app not registering runs and walks (the gyroscopic module?) and even having troubles with iCloud; I have been unable to use iMessages since, even with the help of this support person and various resets, factory resets etc.
The most troublesome I find their conclusion that this "is not Apples fault because I have a cracked screen" and therefor the phone has had impact damage and that also could have caused the mic to give up - this is a crack of ca. 10mm at the bottom corner that has been there for 3 years and never caused any trouble. Now it is a reason to shift responsibility for a problem that is obviously bigger than my cracked screen. As if I don't know what is going on in the world and especially with the combination of the iPhone 7 (and 7+) and iOs 12.
I have had this phone under extended warranty from my provider (EE) for three years, and all this happened three (3!) weeks after my warranty ended. Thank you Apple/EE, I have been an Apple user since September 1998, when I got my first iMac and have been using Apple products in personal/family life and at my companies ever since. I literally spent well over £60k on Apple products in the last 20 years, and have never been disappointed with their customer service, up till now.
That Apple is willing to let something so futile and easy to repair (total bill to Apple for repair £100??) destroy two decades of goodwill and reputation is just utterly stupid in my view.
I will continue to stride for a fair treatment of this issue and will definitely cease to buy Apple products until I have had the customer service that has been so emblematic for this company.