When someone's voice quavers, it becomes unsteady, a little like a stutter. You can describe such a voice as quavering. If you're about to sing on stage for the first time, you might have a quavering voice until you get comfortable. If a big angry person yells at you, you might respond in a quavering voice. Inexperienced public speakers often speak in a quavering way, too. This word (like tremulous) describes the speech of those who are weary, afraid, or faltering.