The antonyms for the word "most mellifluent" would be words that describe something harsh, unpleasant, or grating to the ear. These antonyms could include words such as jarring, cacophonous, harsh, strident, discordant, and dissonant. These words are used to describe sounds that are unpleasant to the ear, often causing discomfort or irritation to the listener. While mellifluent sounds smooth and soothing, antonyms of this word can be used to describe the opposite type of sound. Therefore, it is important to choose the right words to accurately convey your message, whether it be describing something pleasant or unpleasant to the ear.