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!
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):