When you speak monotonously, your voice lacks variation in tone, making it sound boring or flat. The use of synonyms can help you better express this trait. You may describe this as speaking in a dull, droning, or lifeless voice. Alternatively, you may say that someone speaks in a monotone, unvarying, or uniform voice. Another synonym is to describe someone as having a lack of inflection or variation in their voice, or as being monosyllabic. Whether you're describing someone's speech or trying to vary your own, knowing these synonyms will help you better capture and communicate the nuances of speech.