OnePlus
Change Notification Sound auf OnePlus
Every alert on a OnePlus phone plays the same default tone until you assign a different one in the sound settings. This single system-wide sound applies to messages, WhatsApp and most other apps unless the app itself offers its own sound picker.
The setting responsible for this lives inside the sound menu, separate from ringtone and media volume. Changing it there updates the tone used for general notifications going forward, without affecting calls.
To change it, follow this path:
Open the Settings ➔ Tap on Sounds & vibration ➔ Tap on Notification sound and vibration ➔ Choose a notification sound
From the list that appears, you can pick one of the preset tones or add a custom audio file stored on the device. If you only want a different sound for one specific app, such as WhatsApp, check that app's own notification settings first, since some apps override the system-wide sound with their own picker.