Wow, this is a very informatve & helpful posting by Silver Sidelines. I have two of the later Class 42 Warship chassis, one bought complete as D809 Champion, it's been a real pain to fit sound, however by attaching the speaker wires to the feet of the 21 pin decoder adapter (two pins immediately next to the blank indexing space) I have got sound working using a You Choos supplied Zimo MX64 decoder and two 5mm deep sugar cube speakers, quiet at first but altering the volume to 85% is acceptable.
The other chassis was a blue one bought complete 5-6 years ago, this was to replace an old Mainline chassis. I painted the visible part of the chassis green to match my mainline body and it worked fine, in fact for 5 years the body was held in place by slightly wet paint. Previously, I too had removed the mainline motor and ran it as a dummy with the other Bachmann Warship.
Six years later I'm wanting lights & sound in most of my models. To this end I purchased a Bachmann body from e-bay, thinking of proper attachment to chassis and lights. It was D800 Sir Brian Robertson, I was a little put out by the fact that it wasn't the same internally as my Maroon D809. Ho hum, I suppose I'll have to resort to hard wiring, strip board and LEDs with resistors. I wish I'd found this most informative posting previously..................it'd have saved my purchasing a body that's little better that the Mainline one.