Looks like there is no room for sound. A solution would be to put a MyLocoSound Light Diesel soundcard with a 9v rechargeable battery and speaker into a coupled wagon. You would then need only two wires through to the Hercules motor to detect speed and to power the soundcard.
The soundcard has a selectable mode which causes it to run off the 9v battery when the loco is static and then to run the soundcard and recharge the battery when in motion. When the day's run is finished then the soundcard automatically goes into a sleep mode to preserve the battery while the loco is on the shelf. The Light Diesel soundcard has a choice of two engines and five horns including a klaxon. You can see it at
Light Industrial Diesel.
This is also a very cheap way of adding sound to a fleet of diesels, radio or manual controlled. Fit each diesel loco with two wires from the motor to a socket in the back buffer beam. Then fit a matching plug to the two wires from the sound wagon and it can then be used with any of the locos.
Regards
Peter Lucas
MyLocoSound