The best DC diesel sound throttle I ever played with was the original Soundtraxx throttle. You could choose EMD, Alco, GE, FM, Baldwin, and adjust various settings to fine tune the sound. It used an external amp and speakers, and you could get the room vibrating with the bass from an EMD in notch 8 with dynamic brakes going. It had an infrared hand-held unit, and you notched the throttle up and down, just like the real thing (albeit with buttons rather than a lever).
The best DC steam sound throttle had to be PFM's system. It piped taped sounds through the rails to the locos, as well as generating steam and whistle sounds. I believe it let you run sounds through external speakers as well.