I would be concerned too if there was no audio when recording with only one of the cameras. I recommend following these steps:
iPhone 4 or later: Audio issues with speakerphone, FaceTime, video recording,
or people hear noise when I call them
In FaceTime or video recording, verify that both the mic in the receiver opening and the mic near the rear camera are not blocked.
In speakerphone mode and using the phone with the display:
- Face up, verify that the mic in the receiver opening is not blocked.
- Face down, verify that the mic near the rear camera is not blocked.
In handset mode, make sure the bottom mic near the headset jack is not blocked.
- If a protective case is used, try removing iPhone from the case.
- Try turning iPhone off and then on again.
You can find the full article here:
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