Even if your friend claims she's not upset by the death of her pet iguana, her intonation may tell a different story. Intonation means the way someone's voice rises and falls as they're speaking.
Your emotions, your regional accent, or just the particular way you're used to speaking can all affect the intonation of your voice. There's something musical in the way people speak, and intonation describes that musicality. The word also means "producing musical tones," either with your voice or a musical instrument. Another kind of intonation is chanting, or half-speaking, half-singing your words.