I've never heard of that for a headphone jack?? But, if that's the way it works, well there you go..
Have you looked in all the cabinets to make sure there aren't any wires tied up someplace waiting for a receiver to hook to? What about the TV cabinet in the bedroom?
I can see the front and mid speakers being hooked to the same receiver and the bedroom being it's own setup.
You might have to pull the receiver out and see how the A B speaker outputs are wired and go from there. Like I said, I've never had a receiver that you had to switch for headphones.. You just plug in the headphones and it switched automatically, and that was with stereo equipment from the 70's and 80's..
Mitch