monotone has 5 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)
Synonyms, antonyms and the broader and narrower relations for monotone come from Princeton University’s WordNet. Full sources and licences.
of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value
sounded or spoken in a tone unvarying in pitch
“the owl's faint monotonous hooting”