Bongo Fury

Does anyone know how they work? My Bongo already had one when I bought it, but now it's stopped working. The LED on the speaker unit lights up when I select reverse so it's obviously connected, but there's no sound. All the sensors across the rear bumper are clean.
Any thoughts?

They work by using echo location using an ultrasonic tone burst from the the "sensors" which is then reflected back by an obstacle the time duration between sending the pulse and receiving the echo gives the range.
The one you have sounds like an addition as the fitted ones has a pair of LCD displays giving the range for each sensor individually.