I have an LGB 22185 Disney Mogul and would like to add sound. I have ordered a LGB American Steam Sound Unit. Today I opened the tender just to check out what I was dealing with before the sound unit arrives. Theres is a plug (looks like the same plug as the lighting connector on the rear of the tender) inside the tender on the circuit board that is not used. It looks like the PC Board is etched "spkr" is this intended for a sound unit to plug into it? If not what is it for?