The word "mutterer" refers to someone who speaks in a low, indistinct voice. Antonyms for "mutterer" would be those who speak loudly and clearly. Some possible antonyms include assertive, eloquent, outspoken, vociferous, and articulate. These words describe people who speak with confidence, clarity, and purpose. They are capable of expressing their ideas and opinions with conviction and persuasiveness. Unlike a mutterer, who may be difficult to understand or engage with, these individuals are adept communicators who can capture an audience's attention and inspire action. In summary, antonyms for "mutterer" characterize people who speak with conviction and clarity, instead of mumbling or speaking indistinctly.