ESU Class 121 DMU sound decoder and speaker.

Below is a list of the Function keys and their assignments applicable to this model.
F0. Lights on /off
F1. Sound on/off
F2. Horn 1, two tone
F3. Horn 2, single tone
F4. Driver/Guard communication buzzer
F5. Door slam
F6. Guards whistle
F7. Rail joints
F8. Flange squeal
F9. Shunting mode, Press once the loco goes half speed, press it again and you are back to normal control.
F10. Acceleration/Deceleration speed over ride.
F11. Additional horn
F12. Double door slam

ESU PDF sound