For Digitrax Sound decoders, you would need a Digitrax PR3 device. The PR3 has several useful features, the main two being a sound loader for Digitrax sound chips and a computer interface to your DCC system (allowing other software such as JMRI, RocRail or TrainController to operate your layout, or tools such as LocoNet Checker monitor the loconet traffic). The latter use can justify a PR3 on its own, so it may be worth buying even if you later decide that changing sounds isn't that simple.
For advice/help, I recommend the DigitraxSound group on Yahoo!. There you will find a number of helpful people, and references to a number of user-developed toolkits which are a considerable advance on the information, tools and documentation from Digitrax.
http://groups.yahoo.com/group/digitraxsound
However, as a end-user loadable sound decoder, Digitrax decoders are low-cost and low-performance. So, my personal view is that they are not worth the effort to load your own sounds. The most capable sound decoders which are end-user adjustable are probably from ESU and Zimo. Each maker requires their own special hardware for loading sounds. Developing your own sounds for any make of decoder is not a trivial process, so you might want to investigate how this is done, how much work it might be, etc.. before spending money on expensive sound loading hardware.
- Nigel