I added a couple of sound decoders a while ago, and have been very impressed with how the bring the railway to life. I decided to add a diesel one and picked the Class 57.
Youchoos has a guide to installing a Zimo MX660 in the Farish class 57. Link here.: YouChoos
The guide calls for total chassis stripdown, then removing metal to make room for the speaker. I chose to file the chassis rather than grind, and it only took a few minutes. Then add decoders wires to the motor, and stick the decoder onto the top of the chassis (kapton tape then double sided tape). Very quickly I had the chassis running, with sound.
For lighting: my decoder is back to front in the chassis (I have the fan as "front") so the F0 headlamp outputs are OK for the tail lights. I've configured function output 1 as the rear headlight, and function output 3 as the front headlight. To enable these program CV33=17, and CV34=6.
Great result; I suspect I will be wanting more of these!