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)
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”
Synonyms, antonyms and the broader and narrower relations for monotone come from Princeton University’s WordNet. Full sources and licences.