There is no sound when Xiaomi's mobile phone locks the screen.

1. Check the volume setting of the mobile phone: make sure that the volume setting of the mobile phone is in a normal state, not in mute or vibration mode. You can press the volume button or adjust the volume in the settings.

2. Check the application notification settings: make sure that relevant applications are allowed to send notifications in the mobile phone settings, and check the application notification settings.

3. Check the screen lock notification settings: find "screen lock and password" or "screen lock display" in the phone settings, and check the screen lock notification settings.

4. Check Do Not Disturb mode: When the phone is in Do Not Disturb mode, the screen lock notification will be muted.