Items used:
ESU 58429 LokSound 5 21-MTC decoder with DCCSound 1st series X class sound
DCCSound Premium twin driver sugarcube speaker
The inside of the model is accessed by removing the coupler boxes and four screws under the chassis (one at each corner). There is a lighting wire connecting the body to chassis, so take appropriate care when removing the body.
The decoder is inserted as shown below, noting the 21 pin socket on the decoder faces DOWN, such that the decoder is mounted UPSIDE DOWN.
Reassemble the model by fitting the body, screws and couplers. Blu-tak is used to keep the sanding lines away from catching on the chassis, the trickiest part of reassembly.