mouthpiece has 3 distinct synonyms spread across 3 of its 6 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 part that goes over or into the mouth of a person
“the mouthpiece of a respirator”
an acoustic device; the part of a telephone into which a person speaks
a spokesperson (as a lawyer)
Synonyms, antonyms and the broader and narrower relations for mouthpiece come from Princeton University’s WordNet. Full sources and licences.
(especially boxing) equipment that protects an athlete's mouth
the tube of a pipe or cigarette holder that a smoker holds in the mouth
the aperture of a wind instrument into which the player blows directly