The phrase "in a monotone" refers to speaking without changing the pitch or intonation of one's voice, often in a dull or unexpressive manner. There are several synonyms for this phrase that can be used to describe someone who speaks without inflection. Some examples include "in a flat voice," "in a droning tone," "in a deadpan manner," "monotonously," and "in an unvarying cadence." These words can help to add variety to one's writing and prevent overuse of a single term. By using these synonyms, writers can convey the idea of someone speaking in a monotone in a more vivid and descriptive manner.