Here's what I'd like to do for a sound effects system:
Have multiple soundcards (or one 'big' card which appears as such),
and one application playing multiple (un-related) sound files simultaneously (or multiple instances of an app each playing one sound file each - it doesn't matter) asynchronously of each other.
Arrange for the various sounds to appear on more than one output, at different levels on each, and be able to vary the level of each sound at each output as the sounds play (each output would obviously consist of a 'mix' of the various sounds being fed to it at their respective levels at that time).
The net result of all this is multiple sounds which appear to 'move' amongst the outputs, independant of each other.
Windows only officially supports one output sound device at once. That means only one output can be used at any one time. The only way to get around this is if an application (Winamp, Teamspeak, and probably others I don't know about) allows you to manually select a specific output device, in which case you can set that particular program to use a non-default sound output device.
For what you want, you'll need a highly specilized program, and I don't know if any exist to the degree you want. Its possible if a program allows multiple instances of itself AND allows you to select a non-default output, you could approximate what you want. [IE: Using WMP for the default output device, Winamp for another, program X for a third, etc, but most programs like these don't support multiple instances...]