"Full-toned" refers to a sound or voice that is rich, deep, and resonant. Antonyms of "full-toned" could include "thin," "weak," "shrill," "tinny," "hollow," "lacking depth," or "unresonant." A thin sound may be feeble or unimpressive, with little depth, while a weak voice may lack power and volume. A shrill sound might be piercing or high-pitched, while a tinny sound may be metallic or sharp. A hollow sound may lack warmth or resonance, while an unresonant sound may be flat or lack clarity. By understanding these antonyms, one can better appreciate the nuances of sound and voice, and how they can convey different emotions and meanings.