What I would do it try and find out what type of engine is used in them and how many cylinders. Then look for a decoder that has that type of engine and number of cylinders. That should get you close. Since they would not have wanted to make a lot of noise, turn the sound down on the engine or exhaust sound. You should be able to find a bell and horn sound to suite you. I favor Tsunami decoders.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
You may need to download this particular engine sound yourself, or find someone to do it.
For example, you could get a Digitrax Sound Bug, and use a PR3 interface to load the RDC "sound project" into it. The Sound Bug is a sound-only decoder, so you would need another motor decoder along with it. The Sound Bug comes with a speaker. Even though you have to get 2 decoders, this is still probably your lowest cost option.
Other manufacturers make downloadable decoders, but I don't know if anyone has come up with RDC sounds for them. Someone else?
It takes an iron man to play with a toy iron horse.
which is the best sound decoder for the budd railcar?.