voice has 13 distinct synonyms spread across 5 of its 13 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. Its one recorded opposite is devoice.
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 distinctive quality or pitch or condition of a person's speech
“A shrill voice sounded behind us”
the sound made by the vibration of vocal folds modified by the resonance of the vocal tract
“a singer takes good care of his voice”
a sound suggestive of a vocal utterance
“the noisy voice of the waterfall”
expressing in coherent verbal form
“the articulation of my feelings”
a means or agency by which something is expressed or communicated
“the voice of the law”
something suggestive of speech in being a medium of expression
“the wee small voice of conscience”
(metonymy) a singer
“he wanted to hear trained voices sing it”
an advocate who represents someone else's policy or purpose
“the meeting was attended by spokespersons for all the major organs of government”
the ability to speak
“he lost his voice”
(linguistics) the grammatical relation (active or passive) of the grammatical subject of a verb to the action that the verb denotes
give voice to
“He voiced his concern”
Synonyms, antonyms and the broader and narrower relations for voice come from Princeton University’s WordNet. Full sources and licences.