Nouns

(n)monotone, drone, droningan unchanging intonation
(n)monotonea single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

Adjectives

(a)monotonic, monotoneof a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value
(s)flat, monotone, monotonic, monotonoussounded or spoken in a tone unvarying in pitch“the owl's faint monotonous hooting”