Hello OlivierV9000,
The first thing we would suggest would be to power cycle the iPhone by pressing and holding either volume button and the side button until the power-off slider appears. Drag the slider and wait 30 seconds for the device to turn off. While simple, this can refresh the iOS, apps and connections. More information is available here: Restart your iPhone - Apple Support
If the issue continues, we would test the microphones using the steps outlined here: If the microphones on your iPhone aren't working - Apple Support
If all the microphones seem to be working, then the next step would be to try to isolate the problem. Does it happen in all apps that record videos? If it is only one app, then follow the steps here to remove the app and reinstall it: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
Cheers.