The document shared with AASPs states that some iPhone 7 and 7 Plus users may experience mic failure after updating their phones to iOS 11.3.
They may also see the speaker button grayed out during incoming and outgoing calls. Other symptoms include no audio during playback for voice memos or videos creating by the affected iPhones.
The document describes the steps to troubleshoot the issue, such as disconnecting any Bluetooth or other audio accessories and running audio diagnostics. The document states that if the device is out of warranty it may be possible to receive an exception, although that may vary on a case by case basis. It’s unclear if that means a full unit replacement or a repair to resolve the issue.
At this time there doesn’t seem to be a great number of affected devices. But have you seen this issue on your iPhone 7? Be sure to reach out to Apple Support or stop by your nearest Apple Store or AASP.