Wi wing, you are wrong, I have been flying the A320 for many years for airlines all over the world and have never seen anyone don oxygen when the other pilot leaves the flight deck. His breathing could have been picked up if he was wearing his headset with boom mic. The major airline I worked for in Europe always required you to have your headset on throughout the flight. Sure there might be a highly complicated scenario at play here, but simple facts are this guy has a history of mental illness, took actions that crashed the aircraft and prevented any other crew accessing the flight deck. Sometimes 1+1 does equal 2.