voiceless has 8 distinct synonyms spread across 4 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.
deprived of the rights of citizenship especially the right to vote
“labor was voiceless”
being without sound through injury or illness and thus incapable of all but whispered speech
Synonyms, antonyms and the broader and narrower relations for voiceless come from Princeton University’s WordNet. Full sources and licences.