Samsung
Custom Notification Sound per App auf Samsung
One UI lets you assign a separate notification sound to almost any app, but the option only shows up if the app itself registers distinct notification categories with the system. Apps that route every alert through a single generic channel will not offer a sound picker at all, even though the setting exists in the menu.
To check or change the sound for an app, go to Settings ➔ Apps ➔ pick the app you want ➔ Notifications. Inside that screen you get a list of categories the app has registered, such as messages, calls, or reminders. Tap a category, set the sound, and turn vibration on or off for that category specifically.
If an app only shows one combined switch instead of several categories, the sound field usually stays locked to the system default. That is a limitation of the app, not of One UI. Reinstalling the app or waiting for an update that adds proper notification channels is the only real fix in that case.
Keep in mind that Do Not Disturb and app-specific mute settings still take priority over any custom sound you assign here.