I don't know if Kato planned this model to be easy to convert to DCC, but it seems so. I converted several D51, but this is the easiest one and has a lot of space to add sound also.
The weights in the tender are not occupying all the space and you can add a SD10A decoder and a small speaker from D&H. I purchased them in www.decoders.es
If using SD10A decoder, first you need to solder some small cables to the SUSI pads to connect it to the D&H programmer. I couldn't find a specific D51 sound project, but some steam projects have similar sound to the original one as I could check looking for videos in Youtube.
This custom board you see between the decoder and the programmer is just to help with the connection and testing of the sound decoders. I made it to be able to connect the sound decoders with screw terminals, and it has also a speaker. I think the design of the D&H programmer is awful when referring to the connection, programming and testing of sound decoders. It is missing screw terminals and a test speaker. It's not a cheap device and connection of decoders is horrible so better make a small helper board like this one.
Once the sound is programmed, we can come back to the decoder installation. Here you can see were to solder the current wires, red and black ones:
In the following photos you can see how it fits in the tender. Remember to isolate the decoder with Kapton tape and pass motor wires (orange and gray) throw the existing hole like in the photos:
Finally, cut the motor wires (or remove the cooper plates used by the motor to pick up the current) and solder them to the orange and gray cables of the decoder. Ready to test:
I found using this decoder that the locomotive was not running smoothly, it was instead advancing in fits and starts. This was fixed changing CV56 (Motor proportional controller) to value 0.
This CV has something to do with what Doehler & Haass calls "Super Soft Drive". It seems a kind of motor auto regulation, doing some pauses between current pulses to the motor and measuring it in order to adjust the pulse width (or something like that....). But the fact is that it drove me crazy until I could find a clue about it and deactivate it.