As a rule, the "C" button is for another set of speakers...not a "combination" of "A" & "B" speakers together.
If you want to play both "A" and "B" speakers simultaneously, just press the buttons so that both are in the "On" mode. The "C" button is actually for a third set of speakers and on most TTs isn't used. In theory, if you had a separate bedroom area, you could have "A" for living area, "B" for outside and "C" for the bedroom. To play both inside units (living area and bedroom) you would press the "A" button and then the "C" button...both "A" and "C" speakers would then have sound coming out of them.
The reason the "C" button is humming/static, right now, is that there isn't a load on the outputs and you are getting feedback through the other speaker channels. That's the way they are usually
set up; that doesn't rule out the possibility that yours is wired outside the norm
Now the one that doesn't shut off (outside "B" button) whether in the pushed in (On) position or out (Off) position, is most likely a malfunctioning unit...you could check by turning both A and B speaker groups off. You should then get no sound at all from any speakers. If you are still getting sound out the B speakers, then it is the unit's internal B switch that is bad. If you get no sound at all with both turned off, but, you get sound outside with the A (inside) button on, then you have crossed wired, most likely, speaker wires where the B speakers are most probably wired together, or , shorted together so that the B speakers are being powered by the A switch along with the inside speakers.