Items used:
ESU 58429 LokSound 5 DCC 21-MTC decoder
DCCSound Premium speaker for the SRM L class
Disassembly is relatively easy, with just the couplers needing to be removed. Then the body lifts up, off the chassis.

Next, the DC plug socket is removed and the decoder plugged in.
The capacitor, "C12" is removed from the SRM PCB as its capacitance is too high and interferes with the decoder's BEMF control of the motor. This capacitor is wired in parallel across the motor, so removing it does not impede the motor's function. The inductors can stay.
Next, the factory speaker and enclosure are removed, unsoldered from the SRM motherboard.
The new speaker is placed where the old one was, and is screwed to the chassis, using the old screws. The speaker's wires are soldered to the SRM motherboard, but could instead be soldered to the decoder directly.

The body is refitted and the model tested under DCC power.