I noticed that in Infinite Flight on my iPhone XR on iOS 13 and on my iPad Pro 11’ 2018 on iPadOS 13 I can only hear the sound from ATC. But if I pull down the Notification Center, the Control Center or if I activate the app switcher and go back to Infinite Flight there is sound again but after a while it lags for a second and the sound is gone again. My Devices are not muted.
This is a know issue with iOS 13 and iPad OS 13, the issue will be fixed in future updates of Infinite Flight but for the time being there is nothing you can really do.
Known issue, I have it and so do others that have reported it. Not sure if there is a fix. Probably something to do with IOS13/IPADOS13 being BETA software.