lower one's voice; speak under one's breath两个都可以。
例句：
那位男子靠近了一步，压低声音。
The man moved closer, lowering his voice
《柯林斯高阶英汉双解学习词典》
2
不知怎么的，他觉得自己必须得压低声音。
For some reason he felt constrained to lower his voice.
希望对你有帮助！
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