1) Remove coupler chain pin from coupler box on both ends of model
2) Remove 4 body mount screws and coupler assemblies from underside
3) Wiggle body vertically off chassis
4) Remove 21-pin dummy plug
Fit a 23mm speaker for mediocre sound quality, or
Fit a premium DCCSound speaker for far superior sound.
Option 2 requires making some cuts to the chassis with a Dremmel tool and cutoff wheel. The effort is worth it! Tips when removing chassis material:
1) Be careful not to cut the wires along the top of the chassis
2) Cut down from the top of the chassis by 13mm, enough to package the speaker. This is roughly in line with the top of the gear tower.
3) There is a small screw fixing the PCB to the chassis. Maintain the chassis material around the screw hole as the small PCB will need to be refitted.
6) Test fit the decoder and speaker
7) Test operation and then reassemble body back on chassis along with body screws and couplers.
Here's a great video guide from Luke Towan (using the older 23mm speaker):