TRB,
This is going to be kinda difficult to do if you're not going to add TMCC... There is the Railsounds Commander... But I've never used one (but GunRunnerJohn has!).
What I would do to simply add sounds to a conventional locomotive is just order the True-Blast Plus sound system for the RS3, it'll be close enough (than any EMD-sounding unit) and you don't have to change the control system. I found it by Google, it is Bachmann part number 00253.
Check out some videos on Williams True-Blast Plus.
Thanks and good luck!
- Mario