It’s not. These are UX design principles. Media sounds are for user requested sounds, not app initiated (and unexpected) notification-like sounds. It’s correct for DnD to allow the former, for example the Youtube app, or a media player app, since in those app the user is the one who actually taps “Play” to initiate the sound; those apps also usually provide a mute option. In Monzo’s case, the app just decides to play a sound, at an unexpected time, as loud as it can, without providing any control to the user over said sounds. That’s bad UX design. DnD’s purpose is to stop unexpected sounds, not expected sounds.
But the point I was making in the OP is that, if Monzo insists on its app to sing to us, then it should also provide control to the user over those sounds. It’s the only banking/payment app that added sounds, so it should also add the option to turn them off.
On a personal note, I see zero reasons why a banking/payment app should make any in-app sounds at all on manual operations. It clearly bothers people without actually helping anything, really, since if I was visually impaired then I wouldn’t be able to use the app in the first place anyway, and if I wasn’t visually impaired then I could see the result of my manual action on screen. It’s a banking app, not a media player or game. Picture Windows or MacOS making bubbly sounds when opening windows that I have no control over and Microsoft/Apple telling me “Well, just mute your volume if you don’t like it”.
At most, not that it would serve any purpose, Monzo could borrow from more solid mobile UX design principles and trigger an OS notification on manual operations such as send/withdraw/etc. That would provide the user some control, as the user can deal with the sounds/vibrations/notifications at the OS level with OS settings … but they’d still be worthless notifications.