Also if one has SBLive (or two even) and uses the "KX Project" drivers, it is possible to build virtual sound devices and route the signals within the driver (also apply filtering and tone adjustment etc) in real time:
It's complex but could work for intercom in software even. It can map any of the soundcard in/outputs to anything, and you can add a few sblive's even - those are like eur 30 or less.
http://kxproject.lugosoft.com/
//Tuomas