Well, with some effort and Linux guru skills, you could probably whip together an intercom system similar to DjScribble's suggestion, yet without a proprietary PBX system.

I'd explore Asterisk PBX http://www.asterisk.com and the VOIP Wiki at http://www.voip-info.org

I'd take a look at:
http://voip-info.org/tiki-index.php?...20phone%20door
and also
http://voip-info.org/wiki-Asterisk+paging+and+intercom

which whould inevitably lead you to:
http://www.digitalacoustics.com/lanplay.htm
Which sound's like exactly what you need without any Linux hacking and hardware manipulation.
I'd personally try to figure out how to do it with Asterisk, and then plan to stage out there current phone system into VOIP, but that is just me...

Hope this is helpful, I can't believe I stubled upon an actual VOIP intercom system, but then, I have a bookmark collection of VOIP stuff that is incredible since I'm hopefully going to be migrating my office over to some kind of VOIP PBX...

Well, elrey103, you've managed to get me at my most helpful on this board, now I better stop while I'm ahead.

Good luck,
Dhej