Oh, This is a nice thread! I'm using Bass already some months now (great product btw) as a multiroom music server (=client/server app setup).
Virtual Sound Card/Virtual Audio Cable is indeed what I also can use when eg I play a movie on my client, and that the sound is streamed to my music server. The music server sends it to one of sound cards and amplifies it to one of my room speaker.
Valerio: Do you know any example of Windows Messaging and Sound buffers? No problem with synchro? Hard to implement, you think? (Or I should wait until Virtual Audio Cable is available in Bass )