Monsieur Meursault's tone is generally dispassionate, unemotional, impassive, unperturbed and self-possessed.
Examples that support these adjectives include the novel's opening lines:
Mother died today. Or maybe yesterday; I can't be sure.
The death of a parent is usually a watershed moment in a person's life, but Meursault is virtually unmoved by her death and only feels slightly self-conscious when he realizes that people are judging him for his lack of emotion.
Even when Meursault kills the Arab on the beach, his first thought is:
I had shattered the harmony of the day
which would, by most people's estimation, be a hugely understated reaction to taking another person's life.
Even when he faces his own impending execution, Meursault's tone is rather expressionless. He says,
I opened myself to the gentle indifference of the world.
In a very telling confession that immediately follows this line, Meursault finds the indifferent world much like himself.