To be struck speechless or choked up (unable to speak from shock, emotion, or surprise).
コンサートで彼の演奏を聞いて、思わず声を呑んだ。
I was so moved by his concert performance that I was left speechless.
あの場面を見て、観客の多くが声を呑んだ。
Seeing that scene, many in the audience were left speechless.
突然の知らせに彼女はしばらく声を呑んだままだった。
She was silent for a while, struck speechless by the sudden news.
To hold one's tongue; to swallow what one wants to say and remain silent deliberately.
上司の前では批判せず、私はただ声を呑んだ。
I held my tongue and didn't criticize in front of my boss.
感情的になりそうだったので、彼は声を呑んで話を続けた。
He swallowed his words and continued talking because he was about to get emotional.