Do Lufthansa, or EU regulations require that when only one pilot is in the cabin, he dons his oxygen mask...in the event of sudden decompression. I believe that FAA regs require it for US airlines. Is there a mike inside the oxygen mask? If the co-pilot was wearing a mask...maybe that’s why we hear no sounds from him?
I’m sorry, I don’t know the rules over there. If he had a mask on, he still could have answered.
They can hear his regular breathing throughout, and he had to manually initiate the descent, which he apparently did right after the pilot left the cockpit. Supposedly he said nothing else to the pilot, but he was definitely conscious and in control of the plane.