If you're still not able to record a video with sound on the rear camera of your iPhone, we'd recommend that you contact Apple Support directly. You can do that here:
Contact Support
Cheers.
Hello and welcome to Apple Support Communities, nadinefrompoughkeepsie.
If I understand your post correctly, when you record video with the front camera there is sound but recordings taken with the rear camera have no sound. I'd like to help.
Follow the steps in this guide:
If the microphones on your iPhone, iPad, and iPod touch aren't working
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Take care.
Yes, that’s right. And I followed all the steps you suggested and it still doesn’t work
Rear microphone on iPhone 8 doesn’t work.