Where do you mount these decoders? In cars? In locos? Under the layout?
Jared, it depends on the use. If you take a look at the Soundtraxx "Sound Car" flyer, it lists all the sounds. Most it seems would be mounted in rolling stock (flat spots, brake squeal, clickity-clack, generator). The decoder also has two lights and two hyper-effect leads, so I can see it mounted in a caboose with say brake squeal and lights for the conductor's desk and marker lights (most likely fed into fiber optics). Lighted passenger cars with generator sounds would be a good use too.
I don't know if they could be mounted stationary, but some sounds would lend themselves to it, especially flange squeal, which you would want from the whole train, not an individual car.