Seems to me that many of the applications of audio explored in this newsgroup
1) revolve around proprietary, closed systems 2) are limited in what they can do or 3) form separate subsystems that are not well integrated with the HA system as a whole.Here's an approach using audio matrix mixers with voltage controlled amplifiers (VCA's) that is more general and can integrate most all audio parts of a HA system.
The mixers I use happen to be made by Ivie. I've been satisfied with the quality of their internal VCA's and their equipment in general. They are often available through eBay for dimes on the dollar. But one could also make them from scratch and(or) forego the volume control and easily build switching-only matrices with relays. (There is often/usually more than one way to control audio volume).
Also revised is a discussion of control hardware for 'ActiveKnob' control:
HTH ... Marc Marc_F_Hult