How do I record the audio when using screen capture on iPhone?

I experienced the same issue on an iPhone 8Plus until I read the linked instructions below. The answer is found here —> https://support.apple.com/en-us/HT207935

Once you read through the above simple instructions, you will see how to choose whether to include sound with the screen capture, or no sound.

Hello DonnaDenise.

I had this issue with recording on my iPhone. What you need to do is when clicking or 3D touching on the screen recording option/button, before you press start recording click the "Microphone audio" button, this will then record the audio on screen.

Hope this helps!