Here’s another reason why this sound effect thing is bad: it ignores the silent setting on my iPhone.
When you turn on silent mode on an iPhone, that affects notifications and can be used by apps to decide whether to make audio happen (Instagram, for example, does this, which you can override by tapping a video with audio). The “media volume” is separate to the ringer volume.
If you have the app open when making a payment (now more common thanks to the 3D Secure implementation) it makes the noise from the app, rather than from the notification, meaning even if the phone is on silent, Monzo makes the noise. If the last media thing you had open you had the media volume set pretty high, it’s pretty jarring and unpleasant.
So, in short, the app should never EVER make a noise. Have the notifications do that as they already do. Since notifications still appear when the app is open, that should be all you need to do.