OnePlus
Set a Custom Notification Sound auf OnePlus
OxygenOS ships with a fixed set of ringtones, and by default every new message or app alert draws from that same list. If you want a personal mp3 or wav file to play instead, the system needs that file copied onto internal storage first. Without this step, the custom sound option stays empty no matter how often you open the sound settings.
The setting lives inside the general audio menu, not in a separate media app. Once the file is on the device, you pick it up from there and assign it as your notification tone.
Menu path
Open the Settings ➔ Tap on Sounds & vibration ➔ Tap on Notification sound and vibration ➔ Tap on Ringtone from internal storage ➔ Choose a custom ringtone
After confirming your choice, the new sound applies system-wide to standard notifications right away. If you only want a different tone for one specific app, such as a messaging service, check that app's own notification settings instead, since some apps override the system sound with their own picker.
Keep in mind that very short or very quiet audio files can be hard to notice next to a phone call ringtone, so test the volume before relying on it for important alerts.