I found this comment by user rtardxpress on an apple forum:
solution! - turning off audio zoom on iphone 12 pro max when recording video!
so the audio/microphone levels change as you're recording video, and then switch between wide, ultrawide, and telephoto cameras
after screwing around with settings, i discovered that if you turn OFF the "record stereo sound" option in camera settings - this effect seems to go away, and the audio/microphones stay at a consistent level regardless of the camera lens being used!!
just wanted to leave this potential solution here, as ive seen a number of questions asking about getting rid of the microphone zoom "feature"...
and i have been VERY frustrated trying to master audio levels from my iphone clips, when trying to edit them into other video projects