I feel like an idiot, but that's close to my setup that it took me forever to figure out.
I have an A, a B, and a C button. Each is a toggle switch, on or off for that series of speakers.
A is inside speakers.
B is unknown!! Beats me.
C is outside.
So, I can turn on OR off any of the three speaker series. All on, All off, or both or whatever.
But B does nothing.
I know the last campground we were in, the C speaker was really low outside!! I'm old and deaf and it took a big crankup to hear it.