SUMMARY
Currently, it is only possible to customize notification sounds for applications that correctly export all of their individual notification events (essentially only KDE apps). For other apps, there are still "some" settings that can be customized, but these are limited to where and how notifications appear.

The problem is, many third-party apps do not properly respect KDE's "do not disturb" mode, which results in these apps still playing their own notification sound (which goes against the idea of DND), but no popup showing up. A possible workaround here would be to disable notification sounds in these apps altogether, and set a new custom notification sound in system settings (which would then be properly disabled upon entering DND mode), but for this the functionality is currently lacking.

Therefore, I propose to add the ability to "add" a notification sound to any app's notifications, even if that app does not export its notifications in detail. Without any other change, this would result in the app's own notification sound and the custom one playing at the same time, so of course the idea here would be to disable the app's own sound in its settings first.

I'd have a look at implementing this myself as well, but wanted to gather some feedback first if this is something that might be considered for upstreaming.