Android
Change the Default Notification Sound auf Android
Android assigns one shared sound to every notification that does not carry its own custom tone. That is why messages, reminders and system alerts often sound identical, even though the apps behind them are completely different.
The default tone lives in the sound settings, not inside individual apps. Changing it there affects every app that has not set a custom sound of its own, while apps with dedicated notification channels keep their individual tone untouched.
Where to change it
Open Tap on Settings ➔ Tap on Sound & vibration ➔ Tap on Default notification sound ➔ Choose a Category ➔ Select a Notification sound and tap on Save.
The category picker separates preinstalled system tones from music tracks stored on the device, so you can pick a ringtone-style sound or a personal song as the new default.
If a specific app still plays the old sound after this change, that app has set its own notification sound at the channel level, and it needs to be adjusted separately from the app's own notification settings screen.