1. 单调的语调；一成不变的声音
If someone speaks in a monotone, their voice does not vary at all in tone or loudness and so it is not interesting to listen to.

2. (声音或外表)单调的，无变化的
A monotone sound or surface does not have any variation in its tone or colour.

1. an unchanging intonation

Synonym: dronedroning

2. a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)