Since posting this thread I've been able to get ahold of a mixer box - this is what it looks like inside:
I'd love to get my tender working if possible, but I wouldn't know where to even begin. The mixer box probably needs to be gone through, it's in rough shape, and I don't have anything capable of playing the 78 records.
@stan2004 posted:On the flip side, did the engine have any mechanism to block the audio signal from materially affecting engine speed?
These didn't actually come with an engine - you were supposed to pair it with one you already owned, most likely an early postwar Lionel loco.