keep (one's) voice down
To speak in a hushed voice or a whisper; to refrain from speaking too loudly. Shh! Keep your voices down! My parents are sleeping upstairs. I don't care if he was keeping his voice down—he shouldn't have been talking during the movie at all!
keep your ˈvoice down
used to tell somebody to speak more quietly: Keep your voices down, won’t you? The children are asleep.