There is no sound in video playback implemented by AVFoundation elements, in iPad 9 (maybe other as well, but not all models) in iPadOS 16.1 (beta 1 + 2).
The issue is solve-able by explicitly setting AVAudioSession mode (ambient, mix, exclusive, etc.).
But there is no such behaviour in iphone's, so it's a bug, or if it's intended change, then it should be documented.