As many are now aware, I've moved away from 4mm (00 gauge) stuff onto 7mm (0 gauge) nowadays, but I have a 94 year old pal Bill who is still buying various new 00 gauge loco's, and I've DCC fitted a few of his buys and a couple of DCC sound fits, this Class 37 being the latest one. My default choice of sound decoder is Zimo, as I've had past issues with ESU, and normally get sound decoders from Youchoos (others are available!) as I've found the owner John Gymer to be very helpful. Unfortunately, John doesn't as yet do a sound file for the Class 37/4 specifically, so I had to look around, and found on the internet a sound file comparison of 3 different alternatives, so had a listen to them all, and decided to go for the Coastal DCC version, partly since I've dealt with them previously and been happy with their service. I've previously fitted one of the PLUX18 decoders (also to one of Bill's loco's), but not had dealings with the PLUX22 version (as opposed to the common 21 pin versions), and I detailed this in a previous post, so I had expected the PLUX22 to have the same 'Click-in' format as the PLUX18, but in fact it just resembles the normal 21 pin decoder, but with 22 pins! Anyway this is the intended victim, which comes with 2 fitted speakers, so makes a sound conversion quick and simple, though obviously speakers can be changed or added to improve sound if required, space permitting.