How can in-app sounds be disabled even with Silent Mode enabled?

Apple iPhones have a Silent Mode function which mutes all notifications and calls, it does not mute any in-app sounds

Apple iPhone's Silent Mode disables all notification sounds including push notifications, and mutes all incoming call ringing however, it does not mute any in-app sounds.

To mute the in-app sounds and vibrations, these can be controlled directly from within the app:

• Personalise Sound Effects
• Personalise Haptic Feedback