Matysdrummer,
Thanks for giving those steps a try. Next, have you verified that the microphones aren't covered?
You can use the image in the "External Buttons and Connectors" section in the following article to verify where the microphones are located.
iPhone 13 and iPhone 13 mini - Technical Specifications - Apple
Since you've isolated the issue to the Camera app and recording videos, please try to Force the app to close.
If the original problem persists after trying our suggested troubleshooting steps, then give the steps here a try: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won't open.
Please provide us with specifics on what you see for more help if your issue has not been resolved, and we will continue from there.
Thanks for using the Apple Support Communities.
Cheers!