Is there a way to reach the devs to report bugs or make suggestions?
I think device settings are irrelevant. As these are in-app sounds, the app should provide an option to disable them. Even if Monzo respected DND mode it would still not be a solution. One should not have to cripple their device just to make a single app behave. Once again, these are in-app sounds, not notifications sounds, which are already off.
Monzo is the only banking/payment app that has in-app sounds while the app is being used (so far I used Revolut, Starling, Wise, Curve, Crypto.com, Wirex, Halifax, Virgin, MBNA, CapitalOne, Natwest, HSBC, Tesco, Sainsbury’s, M&S, and more).
Given that Monzo devs wanted to go outside mainstream design and introduce in-app sounds, then I’d expect to also have the option disable said sounds in the app.
Personally, I don’t see a reason why a banking/payment app should make any sounds at all. It’s not a mobile game (and even those have sound settings).
There are more. The following was just a cursory search: