General lack of change:
* Monotone: This is the most common word, but it implies a very flat, unchanging voice, often lacking in emotion.
* Uninflected: This is a more technical term, suggesting a lack of variation in pitch, stress, or rhythm.
* Level: This emphasizes the flatness of the voice, as if it's on a single level.
* Even: This suggests a consistent, unvarying tone.
* Steady: This emphasizes the lack of fluctuation or change in the voice.
Emphasis on lack of emotion:
* Impassive: This indicates a lack of emotional expression in the voice.
* Apathetic: This implies a lack of interest or feeling in the voice.
* Indifferent: Similar to apathetic, this suggests a lack of concern or emotion.
* Neutral: This simply means the voice doesn't convey any particular emotion.
Other possibilities:
* Bland: This suggests a lack of excitement or interest in the voice.
* Dull: This implies a lack of energy or liveliness in the voice.
* Flat: This emphasizes the lack of variation and excitement in the voice.
The best word will depend on the specific situation and the desired meaning.