In iOS 14 or later, go to Settings -- Control Center -- More Controls -- then tap + next to Screen Recording.
Swipe down from the upper-right corner of the screen to locate
Screen Recording icon.
To turn on microphone during recording: press deeply on the recording icon
and tap Microphone.
Tap Start Recording, then wait for the three-second countdown.
Tap the red status bar at the top of your screen to stop recording.