Hi honey_fulminar,
We'll be glad to address this with you. Depending on which camera you're using, a different microphone may be in use (your iPhone has multiple microphones). With that said, if you have a case or any other accessories on your device, try removing those to see if this persists, as it could be blocking the microphone.
A definitive guide for this can be found here: If the microphones on your iPhone aren't working - Apple Support
If it happens with nothing on your microphone, and nothing seems to be blocking any of them, reach out to Apple here as you may need to have your device serviced: Get Support
Cheers.