The mounting for the audio switches (and the marker beacon lights and switches), is already in place behind the eyebrow panel plastic. You just drill holes in the plastic to match. I just picked up a spare marker beacon receiver (along with a spare glide slope receiver) on Ebay for $35, although I still need the connectors, switches, and lights. (The switches are absolutely identical to a certain Radio Shack part number.)
Everything works the same as any audio panel.
You can listen to any combination of radios, all muted upon transmit.
The audio "panel" combines all of the audio outputs you have selected to "phones" into one output which feeds your intercom (I just relocated the panel mounted one in Dingbat to clear a slot in the stack for future use), so all you need is a stereo compatible intercom with stereo input to listen to virtually anything that will plug into it by patch cord. Audio wise, it's all seamless and compatible, once you learn where to set all of the different volume controls.
Yes, schematics are the key, and I can probably supply any you need for Cessna panels.
See attached. Marker beacon on the left, intercom in the middle, then transmit switch and audio switches on the right.
