soundness has 3 distinct synonyms spread across 2 of its 3 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 unsoundness.
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 state or condition free from damage or decay
the quality of being prudent and sensible
Synonyms, antonyms and the broader and narrower relations for soundness come from Princeton University’s WordNet. Full sources and licences.
the muscle tone of healthy tissue
“his muscular firmness”