tone has 28 distinct synonyms spread across 11 of its 15 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.
the quality of a person's voice
“he began in a conversational tone”
(linguistics) a pitch or change in pitch of the voice that serves to distinguish words in tonal languages
“the Beijing dialect uses four tones”
a notation representing the pitch and duration of a musical sound
“the singer held the note too long”
the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli
“the doctor tested my tonicity”
the quality of something (an act or a piece of writing) that reveals the attitudes and presuppositions of the author
“the general tone of articles appearing in the newspapers is that the government should withdraw”
Synonyms, antonyms and the broader and narrower relations for tone come from Princeton University’s WordNet. Full sources and licences.