''a sound girl'' means?

basically, sound = good, perfect, nice

a sound girl= nice girl

might mean
1.good looking,
2.good shape,
3.nice to stay with,

hope it is helpful
cheers

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得閒上黎睇睇﹐識多D 英文

A sound girl means a good looking girl.
adj(sound)=sensible,give good results,a girl of sound judgement,gives sound advice.
Not to the effect produced by the music of she as a singer:-I don't like her sound.