Leaked memo from Apple says iOS 11.3 disabled the mic on some iPhone 7 and iPhone 7 Plus models
The memo recommends that authorized repair centers first ask users to disable any Bluetooth devices that might be connected to their iPhone. If that doesn't solve the problem, a series of audio diagnostic tests will be run. If the results indicate a problem with a user's iPhone 7 or iPhone 7 Plus, the repair center can write a ticket to fix the problem. Obviously, those with a device covered by the original warranty have no problem. Those with an affected phone with an expired warranty can still ask the service center to request an exception that will allow the repair to be covered by Apple.
Why iOS 11.3 is disabling microphones on the iPhone 7 and iPhone 7 Plus is unknown, but Apple does feel that in some cases the software update is causing a hardware problem that requires a repair to be made.
While Apple has yet to confirm the issue, if you are having these problems with your 2016 iPhone, you should make an appointment with the Genius Bar inside your local Apple Store. Because of the discounted battery replacement deal Apple is running all year, it might be hard to schedule an appointment. You can also try to have your handset fixed at an authorized Apple retail center.
Several complaints about this issue have been seen in the Apple Support Communities Forum, Reddit, and Twitter.