murmur has 12 distinct synonyms spread across 5 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 low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
a schwa that is incidental to the pronunciation of a consonant
an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves
Synonyms, antonyms and the broader and narrower relations for murmur come from Princeton University’s WordNet. Full sources and licences.