How to be more sure or guarantee screen recording will record the audio on iPad / iPhone?

I am using iPad Pro (first gen) with iOS 12.0.1. If I try to record a game, such as Plants vs Zombies using the iOS built in function of screen recording, I found that often there is no audio recorded. Sometimes it is recorded, if I am "lucky". So I can be playing for 7, 8 minutes, to beat a level, and get ready to post on YouTube, and then found that there is no audio. And then, try for another 7, 8 minutes and see if it is another 7, 8 minutes wasted. I have also tried: start the screen recording first, and then start the game, or the other way around: start the game first, and then start the screen recording. But neither can guarantee audio is recorded. How can it be made to work? P.S. I recorded one clip, and was so happy that I heard the audio while watching the recorded clip of the gameplay, but until about 27 seconds later the audio all of a sudden disappeared. So the recording is again useless. The key point is that the audio can spontaneously disappear.