I found the reversing sensors on my G3xh too quiet and decided to do bit of investigation.
Although the manual includes a description of the reversing sensors it seems that the UK cars do not have this mitsubishi system fitted.
I removed the rear trim and found an aftermarket system had been fitted. I presume this is fitted by Mitsubishi UK when it reaches the UK.
The speaker for the sensors was stuck behind a metal stiffener in the rear of the loadbay. It had no volume adjustment.
The speaker had a long wire connecting it to the control unit and by unrolling this wire I was able to relocate it further forward in the car. Mine is now located under the plastic trim in the side of the floor just in front of the rear bench seat. This has made it much easier to hear from the drivers seat.
If you want to try the same, the only tools needed are a screwdriver for removing the rear loadbay trim and some snips (or scissors) to cut the cable tie in order to unravel the cable.