I'm sorry to hear you're having issues with recording video without sound on your iPhone 7. That does seem quite frustrating, but let's see if we can find a solution.
First, it's important to confirm this is a software issue and not a hardware problem with the microphone on your iPhone. As you mentioned, the microphone seems to be working fine in other apps and functions. To double check:
•Make a phone call to someone and confirm they can hear your voice through the iPhone microphone.
•Record a voice memo or note using the Voice Memos app. Play it back to ensure your voice was captured.
If the microphone is working in other areas, this points to a software glitch likely related to the Camera app itself. Some steps to try:
•Force close the Camera app by swiping up from the bottom of the screen to open the App Switcher. Swipe up on the Camera app preview to force close it. Then re-open the Camera app and try recording video again.
•Update your iPhone to the latest version of iOS. Software updates often contain fixes for Camera and microphone issues. Go to Settings > General > Software Update to install any available updates.
• Make sure to also pay a visit on If the microphones on your iPhone aren't working – Apple Support (UK)
•As a last resort, you may need to perform a hard reset of your iPhone or factory reset and restore from backup. These are more drastic steps, so only perform them if the above suggestions do not work and there are no hardware issues found.
I hope one of these solutions helps to resolve the problem with recording video without sound on your iPhone 7. Let me know if you have any other questions!