So... checking the tender on #5484, it does indeed contain one speaker, one circuit board (completely detached from the foam backing, which is quite hardened and useless) and three wires to the tether plug. No sign of any connection to the bogies.
I strapped the board to the tender as suggested and connected the tether. The sound board makes various strange and at times, rather pathetic hissing, croaking and bleating sounds, so I think its usefulness is at an end.
I’ve coiled the tether out of the way inside the shell for now, and taped over the loco end.
