"unvarying tone in music or speaking, utterance at one unvaried pitch," 1640s; see monotony. OED says use of the word as a noun is peculiar to English.
Monotone is a natural device for increasing the sonority of the voice, so that it may readily fill a large space, and is also thought by some to have a peculiar solemnity of effect. It is much used as an element in chanting. [Century Dictionary]