Interrupt whole house audio with arbitrary source, post-receiver but pre-amplifier
The device you're looking for is a "ducking mixer". Websearch will find public-address hardware that has this feature, and may find a consumer version or circuit diagrams.

It's basically the same idea as automatic volume control, except that rather than reacting to an input becoming louder by fading that input down, it reacts by fading the other signal down. There's usually a bit of delay built into this, so the fading and restoring of the normal/background channel sounds fairly natural rather than having it cut in and out suddenly every time there's a pause in the higher-priority signal (think turning a knob rather than flicking a switch, and waiting to be sure there isn't another word/sound coming a second later before going back to normal operation.

Of course if you want multiple signal sources to be able to interrupt your listening, you either need to mix them before they get to the ducking amplifier or the ducker has to support multiple high-priority sources. The former's probably more common/affordable.

Usually the easiest way to patch effects of this sort into a consumer receiver is to use the "tape loop" outputs and inputs, if your system has them. Switching to tape mode routes the normal signal to the receiver's "tape record out" connection, and routes "tape playback in" to the amplifier. That was originally intended to let folks monitor the signal that was actually being recorded on tape, but it's useful as a general "effects insert" path. (External equalizers also tend to be patched in via the tape loop, for the same reasons.)

Sounds like a fun home-automation project. Shouldn't be too hard to make it work.

("Mr. Smith, please pick up the white courtesy phone. Mr. Smith...")