I want to setup my home audio so that I can inject a source of audio (which could be anything from a doorbell to computer audio) more-or-less "in-between" the receiver and the amplifier, so that audio from that in-between-source would take priority over the receiver's output. Ideally, it would fade down or mute the receiver programming momentarily, too.
In other words, I want a setup similar to how a paging/intercom system might interrupt elevator music in an office building. Frankly, I don't even know the proper terminology for such a thing.
My long-term vision for this includes being able to assign from sources like an AirPort Express to which I would AirPlay an iPhone, and I would expect to hear the iPhone alerts (SMS texts, phone call rings, etc.) to output OVER any other base audio that might be playing from the stereo receiver.
Is there a ready-made device for something like this? Is this possible? Does this even make sense?