silence has 9 distinct synonyms spread across 4 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. Its one recorded opposite is sound.
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 state of being silent (as when no one is speaking)
“there was a shocked silence”
the absence of sound
“he needed silence in order to sleep”
a refusal to speak when expected
“his silence about my contribution was surprising”
the trait of keeping things secret
Synonyms, antonyms and the broader and narrower relations for silence come from Princeton University’s WordNet. Full sources and licences.
keep from expression, for example by threats or pressure
“All dissenters were silenced when the dictator assumed power”