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》

He spoke a few words, his voice low and calm.

他说了几句话，声音低而平和。

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The technology would enable a person talking to someone with a loud, high-pitched, rapid-fire voice to sound similar even if his actual voice was soft, low and slow, for instance.

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This impression is magnified by his rigid demeanor and his baritone voice, which he deploys slowly, at low volume.

NEWYORKER: No Secrets