I also tapped into the Aux Input wiring harness that runs from the head unit to behind the passenger footwell panel. Just set the Android head unit to max volume and you should have no sound issues.
There is a detailed write up on the Disco3 forum on how to find and tap into it. It is the red, blue and black wires on grey plug behind the footwell panel (blue left, red right, black common), or you can pull out the head unit and tap into at the radio end.

