Honestly I think we need two things. A piece of software that can take one stream and put it out many physical outs, IE a Mixer, then to be able to control the volume on those physical outs. So all you really need to do is integrate a software mixer into xlobby. A generic software mixer usually works off what windows see as the different outputs you can use. Like here is an example of how it can be setup:
You use the zones how they are normally used. You can have one song playing at one place and another playing somewhere else. Then there is a button called party time or something like that. This then brings up a different screen and new copy of winamp. It then has the front end that just displays every zone you have and then in the backend it’s based off of a mixer type program. Everything is muted by default. You then start your song and enable the certain zone you want. This then refers to the sound card interface in the mixer and unmutes the interface on the sound card. You then can click again on the zone and it brings up the volume screen for that zone. You change the volume and it changes the volume on the mixer backend. You then can select the next zone and repeat. The zones to sound card reference will be setup in a config file or something. You can actually accomplish all this already through windows but putting them into a program or integrating a mixer in to xlobby would make it so you can do it directly from xlobby.
But the best thing about doing it this way would mean that it would work for any card and you could still use each zone independently or use the party screen for just that, Parties!!