Chas, you weren't hearing things. The audio panel for the jumpseater is repeaded to the channel 9 system on most aircraft in the fleet. The jumpseater is always breifed to only have channel 1 turned up unless they need to hear the pushback, ATIS or interphone comm (FAA ride along), but then to have it turned down for the rest of the flight.
On more than one occasion I've seen the FAA or a jumpseater hose up the panel because they weren't used to the configuration. Guess what, the pax will hear everything tuned in, and it will be a malestrom of audio from every source tuned into the panel including VOR ID, etc. If only the cabin comm was turned up as well as COM 1, then of course you'd have what you had. If I was the Capt. I'd probably turn off channel 9 also if news came forward that the PAX were hearing crew communications as well!
By the way, channel 9 has been around at UAL since at least the late 70's I believe, and it has always been used as a passenger convenience and interest marketing tool. A good one at that I think! It's even included in the Captain to cabin crew breifing at the beginning of crew check in (use or no use).