Xiaomi
Set a Custom Notification Sound auf Xiaomi
MIUI does not limit notification tones to the built-in ringtone list. Any audio file saved on your phone, in any folder, can be assigned as a notification sound, which is useful if you want a short clip instead of a generic system tone.
To set it up, go to Settings ➔ Sound & vibration ➔ Notifications ➔ All ringtones. From there tap Choose local ringtone ➔ File Manager, then browse to the audio file you want and confirm with OK.
The file length matters here. If you select a full song, MIUI will still play it from the start, and a long intro can make the notification feel delayed or get interrupted by the next alert before it finishes. A clip of one to three seconds works best for quick recognition.
If the sound does not show up as expected afterward, check that the file format is supported (MP3, OGG, and WAV usually work) and that it is not stored in a hidden or app-restricted folder. For per-app tones instead of a global notification sound, use the individual app notification settings rather than this system-wide menu.