When it comes to describing speech that is lacking in variation or interest, "in a monotonous voice" is a common phrase. However, there are many synonyms that can be used to convey this same idea. "In a dull tone," "in a flat voice," or "in a lifeless manner" are all similar alternatives. Other options include "in a droning voice," "in a monochrome tone," or "in a uniform cadence." Ultimately, which synonym to use will depend on the context and the writer's personal style, but these options provide ample variety and nuance for describing tedious or uninspired speech.