It seems as though the new sound APIs in Vista (and beyond) were rewritten but still have the same interfaces. That means that old code will still work with the new APIs. However, the one change that was introduced was to make the volume changes per-application instead of system-wide (via the master volume). Here is an article that talks more about that:
It would seem as though you could use this to your advantage by writing your code to work to the old interface, thus making it application-specific. You would need to use it on a Vista or later machine but since it would still work on XP (just not be application-specific) it wouldn't be that big an issue (in my opinion). I don't know of anything that allows application-specific sound settings in XP.