It dates from a time when the secondary definition of sound - 'whole, not damaged or injured' - was more commonly used. (It is this definition that gives us the expression 'of sound body and mind', which is still used today.) Often people will say 'I'm sound' when someone asks 'how are you doing?' We even say 'I'm sound as a pound' = I'm okay, doing well.

Sound means, in this case, healthy/fit/not lame/reliable/based on good judgement Sound reasoning = good logical thinking A sound horse = a horse with 4 good legs! Sound asleep = in a deep sleep In parts of Britain (in the north) they use 'sound' instead of 'well' as in the following example: "How are you?" "Oh, I'm sound, thanks."

"Sound" has completely different meanings as a noun, verb, and adjective. These were originally very different words which now have the same spelling. As an adjective, "sound" means "healthy". It comes from the German roots of English. "Sound" meaning "voice" (a noun) comes from Latin.

In this case the word "sound" has a different meaning. It means "alright" in this context (maybe 好 or 沒事 in Mandarin). It can also be used in a similar way to mean something/someone is healthy (健康 ) or reliable (可靠). Apologies if my Chinese isn't very good!