monotone

(noun) an unchanging intonation. Synonyms: drone, droning.

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

(adj) of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value. Synonyms: monotonic (vs. nonmonotonic).

(adj) sounded or spoken in a tone unvarying in pitch. Synonyms: flat, monotonic, monotonous. Examples: "The owl's faint monotonous hooting."