He spoke in a confidential tone, his voice low.

他低声用隐密的语气说话。

《牛津词典》

He kept his voice low-pitched in case someone was listening.

他压低声音，免得旁人听见。

《柯林斯英汉双解大词典》

In Russia and western countries, having a meal in a public place means you must keep your voice low.

在俄罗斯和西方国家，在公共场所用餐意味着你必须压低声音。

youdao

Roosevelt's voice was very low, almost a whisper.

VOA: special.2010.09.16

Roberta Gambarini sails like an angel on top of a song, while Freddy Cole's baritone voice rides low, full of experience but still sentimental.

NPR: Freddy Cole and Roberta Gambarini in Concert

Another morgue attendant, an Indian-looking man some kind of doctor had come to speak in a low voice to the police officers.

NEWYORKER: I.D.

Finally, my eldest brother spoke in a low voice.

NPR: Literature and Lullabies from the 'Axis of Evil'