Soundlessly means in a way that makes no sound. It is the adverb, so it describes how something is done: a door opens soundlessly, someone crosses a room soundlessly, a cat drops soundlessly onto the grass. Like the adjective it comes from, this is writer's English — you find it in stories, especially where somebody is moving carefully and does not want to be heard, or where a scene is being watched from a distance with the sound somehow removed. Notice how often it goes with movement verbs (move, walk, slip, glide, open, close) and with weeping — 'she wept soundlessly' is a whole picture in two words, grief with the noise taken out of it. In everyday speech people say 'without a sound' or 'quietly' instead. 'Without a sound' is the phrase to reach for if you want the same meaning but natural spoken English.

無聲地、悄然地。文學用語，口語通常說 without a sound 或「悄悄地」。