I'm seeing the same thing after upgrading my iPhone 7 to iOS 12. This article suggests that it's a known issue: https://www.macrumors.com/2018/05/04/apple-aware-of-iphone-7-mic-issue-ios-11-3/
2 replies
https://support.apple.com/en-us/HT203792
https://support.apple.com/en-us/HT203977
https://support.apple.com/en-us/HT201252
If still unresolved, then your device has a hardware failure and needs to be serviced.
Audio device not found after update to ios 12.1