Soundless means making no sound at all. It is an adjective, and it is a literary word — you meet it in novels, poems and careful descriptive writing far more than in conversation. In speech people say 'silent' or 'it made no noise'. So when a writer chooses 'soundless', they usually want the reader to feel the absence, not just be told about it: a soundless flash of lightning far away, a soundless film, a soundless laugh where the shoulders shake but nothing comes out. That last use is worth noticing — the thing is happening, fully and visibly, but the sound has been taken away from it. That is where 'soundless' does work 'quiet' cannot do. It sits before a noun most naturally ('a soundless room'), though it can follow a linking verb ('the house was soundless') and then it sounds distinctly bookish.

無聲的、悄然無聲的。偏書面、文學的用字，日常口語一般說 silent。