These baby monitors are intended to be powered on and off by pressing the volume control, toggling the power.
The receiver speaker covers and transmitter inset shapes (the red heart shape on the girls and the blue triangular shape on the boys) are intended to house a 'power on' indicating led.
The 'volume' control would control speaker volume on the receiver and the microphone sensitivity for the transmitter.
The screw on the back retains the back cover where batteries would be held inside.
The modifiers and materials used in the preview are included with the .blend though not the .obj.