This describes when someone says something softly, so other people won’t overhear, often to surreptitiously give the person instructions.
It’s from the Italian “sotto voce,” which literally means “under the voice.” It has history as stage direction during performances.
Why I Looked It Up
During an episode of Chuck, a character said something to another character quietly, and the closed caption was prefaced with “SOTTO VOICE:” It happened twice in the same episode.
Added on August 28, 2022
From the novel Reamde:
She was pretty sure she heard the word for “Russian” too. But it was difficult to make anything out, since all of the conversations were sotto voce, and anyone who raised his voice to a conversational level was glared at and hushed.
Added on April 30, 2023
From Valerie Plame-Wilson’s book Fair Game:
On the professional front, my colleagues at the CIA tried to respect my privacy and offer their support sotto voce, when they could.
Note the usage of “voce” here, which is Italian for “voice.”