There are a lot of ways to do this, depending on what exactly you want to do.
The easiest way I can think of would be as easy as a relay board with your sound card as the input and a relay selects the room for output (this is simple and has a lot of limitations). Or perhaps a multichannel sound card, but this would depend on your automation software and require an amp for every channel (NCH Swift Sound makes a neat software package, Business Music System, that is capable of handling multizone music and announcements, and can be controlled from other software:
Next step up would be a matrix switch with multichannel output with as few as two channels input, if for example you wanted announcement and music.
Multichannel / multizone audio distribution can get expensive pretty fast.
A professional multizone intercom device like this would work too:
cool, but also not cheap)
Most of the wireless music streamers I have seen are set up for control at the client device, not the server. So you couldn't necessarily interrupt the music to play an announcement (they are more like for streaming audio files, than encoding and streaming live audio). There are live music streamers, but these are more in the professional range and, again, multichannel gets pretty expensive.
Well you're way ahead of the game then. If you're comfortable programming then you could do it without homeseer and talk directly to the AB8SS.
I like using HS though, because you can setup the events and the trigger criteria from the HS user interface, then fill in the blanks with custom code. It's a big timesaver, and the more complex the setup the more it pays off to use HS, or something like it. I have zoned whole house announcements tied into caller id, the security system, plus other miscellaneous items.
I ran a homerun wiring configuration. I prefer it over wireless. Wiring is a pain, but it's a good long term investment.