One one day at Denver having arrived from Vegas on a Ted A-320, I asked the Capt why he turned off the aircraft - ATC audio feed which United broadcast through the entertainment system just before take off.
His response was that sittin' at the threshold he could see the biggest mother f***er of a thunderstorm sitting on our climbout path and he didn't want to frighten the pax when he reported what he saw and called to vector round it.
So his was on before take-off and obviously looking up. Is it like a transponder .. turn on when entering the runway ?