If you use a cover for your iPhone, try removing it and testing the audio recording. If that doesn’t resolve the issue, please do a forced restart.
Force restart iPhone - Apple Support
If the forced restart doesn’t resolve the issue, please contact Apple Support. You need to have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US, please call 1-800-MY-APPLE, or outside the US, consult the support articles below. In Canada, call 1-800-263-3394.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple
Explore Service Locations
Official Apple Support