Bachmann Class 66 Sound Fitting Guide
The Bachmann class 66 is a popular model for sound fitting. Theres plenty of space for a speaker, you can use a single 25x25x7, or a pair of 20x20x7 megabass speakers.
You can also use an EM1 speaker but its important to make sure the lighting wires that run along the roof are taped out of the way.
Find the sound decoder here.