Very interesting article from The Guardian in the UK
Germanwings flight 4U9525: what?s it like to listen to a black box recording? | World news | The Guardian
Once investigators do so, they can find vital clues in the tapes – even in the unconscious sounds emitted by the pilots.
“You can listen for things like breathing rates,” says David Gleave. “You can judge stress levels, whether there’s been a heart attack. Maybe you turn up the pilot’s mic, or maybe you try to detect stress in the voice of an individual. You listen and try to work out if, when they’re silent, is it because they’re getting on with it? Or is it because there’s tension between them, and maybe that’s going to lead to a misunderstanding?”