Repetition. Vocalic cues reinforce other verbal and nonverbal cues (e.g., saying “I’m not sure” with an uncertain tone).
When reading sarcasm is can be really difficult to pic up on these tones. Because sarcasm often relies on the subtle manipulation of vocalic cues it is almost impossible sometimes given the context of the story to understand. Bu that makes it essential to pay attention. You have to not only to what is said but also to how it is said by fully grasping the situation of the novel.