Well designed model from Kernow, the Bulleid Diesel already has a reasonable speaker on board, and a 21pin DCC socket, which
takes a Zimo MX644D sound decoder. Lots of space to spare inside if you wanted to push the boat out.
Just keeping the sample 470uF capacitor that comes with the MX644D, we protect it in Kapton and BlackTack it to the bulk head
The MX644D plugs into the 21pin socket, and off you go! Dead easy!
Just for laughs, we put a dual IceCube 28X18X12-2W behind the bulk-head, which fits very comfortably, so if you wanted to
bypass the factory speaker, you could go for one of these instead, which would certainly be louder.