1.   He was only half listening to the monotonous voice of the teacher.

2.   I can hear it in the monotonous voices of my friends.

3.   She speaks about herself in a monotonous voice, frequently confusing genders.

4.   To avoid a monotonous voice, you should vary your pitch.

5.   But she has withstood frivilous teasing and criticism in the press, where commentators have taken her to task for a monotonous voice and a penchant for conservative dress.