As Ron said, an aircraft intercom might be the answer. Depending on how much you want do from scratch, I've attached an intercom circuit I built several years ago. I ran across an assembly manual for one sold by RST Engineering. Was sitting at home with a broken leg, so decided to build my own, using their input component values, and a lot of my mod's. 4 channels, with one VOX level and one volume level control. The input channels and output amp(s) could be expanded to any number. You could also modify it so each channel had their own VOX and volume control. Sorry about the PDF quality. I had to recover the schematic from an old version of AutoSketch and run it to a PDF printer to make it readable at all.
It worked great, but was stolen out of my car or I would still use it.
Ken