Good thing there's someone else with you on the flight deck!
That 'other thread' says that the 'someone else' is going to be eliminated any time now...
I don't think channel 9 exists today, but I could be wrong.
If only someone had answered that question in post #9 in this thread.
I long suspected that they weren't limited to Com 1
It's connected to the output of one of the jumpseater's audio panels. Any audio source on the panel can be routed to Ch. 9. There is a switch within easy reach of the pilots which can cut it off quickly if something that could unnecessarily frighten the passengers is going on.
25+ years ago I was flying for one of the American Eagle commuters. Customer service letters related to pilot stuff would be forwarded to the respective chief pilot for response. He'd save the better ones. A couple I remember...
1. Flight on a Metroliner. Private pilot passenger wrote in to tell about how unsafe the pilots were. They didn't even do a mag check prior to takeoff.
2. Flight on a Saab 340. Private pilot passenger wrote in to talk about the unsafe altitude at which the pilot's flew. They flew at only X,000' (something well below 10,000'). He knew this because his wrist watch had an altitude function.
3. Flight to BMI (Bloomington, IL). Flight flies the ILS but goes missed and diverts to IND (Indianapolis, IN). Passenger writes in to complain that the pilot should have been able to land and that the pilot never had any intention of landing in BMI. He knew that they should have been able to land because he could see the runway directly below as they went around. He was also upset that they waiting until just "200' above the ground to DECIDE if they were going to land". It was obviously unsafe to wait so long to DECIDE if you were going to land at a HEIGHT of only 200'. He knew that the pilot never had any intention of landing at BMI because when they went around they immediately raised the landing gear instead of leaving it down for another approach. He also complained that they should have diverted to CMI, not IND, as that would have been more convenient for the passengers.
So, yeah... Passengers, even those who are pilots, can have some serious misconceptions regarding what's really going on.