'Sound' gained its final '-d' in Middle English — a phantom letter never present in Latin 'sonus' or French.
Vibrations transmitted through air or other media that are perceived by the ear; to make or emit a noise.
From Middle English 'soun, sound' (noise, a thing heard), from Anglo-French 'soun' and Old French 'son' (sound, noise, musical note), from Latin 'sonus' (sound, noise, tone), from PIE *swenh₂- (to sound, to resonate). The final '-d' in English 'sound' is an unetymological addition — it was not present in Latin or Old French and was added by analogy with other English words. This 'sound' (noise) is unrelated to 'sound' (healthy) or 'sound' (a body of water), which have entirely different origins. Key roots: *swenh₂- (Proto-Indo-European: "to sound, to resonate").
English 'sound' has a phantom letter. The '-d' at the end was never part of the Latin 'sonus' or Old French 'son.' It was added in Middle English by analogy — English speakers tacked it on, perhaps influenced by words like 'ground' or 'round.' The French word is still just 'son,' without the extra consonant