There is a point at which both sounds overlap fully. Audio that originates outside the story is non-deigetic. Functions as surrogate for natural motion. Functions to establish context for what follows. Refers to lighting style.
High key is bright and cheerful without shadows.
Low key is heavily shadowed, mysterious, even depressing. Can be on camera or voice-over. A sequence of actions that are causally linked linked. A new element that comes into an on-going plot. Moves it in a new direction, one that would not be telegraphed to the audience ahead of time.