Haskell's On30 RTR model has room for a decoder in the smokebox, and a speaker in the coal bunker. If you're prepared to rework the model, options for the speaker(s) may include the water tanks (with weights removed) or in the firebox. To access the 8 pin plug, remove the small hatch cover on the underside of the loco, near the 2nd driver. Once removed, use a jeweller's screwdriver to remove the screw inside the hatch and once removed, the boiler top is free. Plug the decoder in, careful not to yank on the red and black pickup wires. If they break, the loco must be completely disassembled in order to resolder the wires to the pickups (see below!).
Solder the decoder's brown speaker wires to the PCB that the decoder plugs into, where the purple wires connect. These purple wires run from the PCB to a small PCB in the coal bunker.
Remove the coal bunker top (it slides vertically up and out), and pull the small PCB through and remove it from the purple wires in the bunker. Then solder the 16x25 mm LokSound 4 ohm speaker to the two purple wires. Fit the white speaker enclosure over the back of the speaker and seal any gaps with Krsital Klear or equivalent. Once dry, place the assembly in the coal bunker and refit the bunker floor and boiler top into position.