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sotto voce (soh-toh voh-chi), adv. or adj., in an undertone, in a low voice

When you say something sotto voce, you say it very quietly. If you're unsure of the lyrics, you can also sing a song sotto voce.

This handy Italian phrase can be used as an adverb: "'Don't look now, but there's an alpaca behind you,' she said sotto voce."

It's also fine to use it as an adjective: "I liked the sotto voce part of your karaoke performance best."

Sotto voce, literally "under the voice," comes from the Latin words subtus, "below," and vocem, "voice."
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This is where I saw it, a couple days ago:

I wrote the column for Life letting readers know who I was. It appeared. At the time it seemed an unexceptional enough eight hundred words in the assigned genre, but there was, at the end of the second paragraph, a line so out of synch with the entire Life mode of self-presentation that it might as well have suggested abduction by space aliens:

"We are here on this island in the middle of the Pacific in lieu of filing for divorce."

A week later we happened to be in New York.

"Did you know she was writing it," many people asked John [the author's husband], sotto voce.

Did he know I was writing it?

He edited it.

He took Quintana [their daughter] to the Honolulu Zoo so I could rewrite it.

He drove me to the Western Union office in downtown Honolulu so I could file it.

(from Joan Didion's memoir, "The Year of Magical Thinking")

Other examples:

I did tell you, he says, sotto voce.
The Guardian Sep 15, 2018

At all hours, young men invite you sotto voce to a “coffee shop” à la Amsterdam.
The Guardian Aug 30, 2018

Root, according to a witness, leaned toward friends and suggested sotto voce that they all cut away for cocktails.
The Devil in the White City

Cockburn produced a special edition of the Week devoted to the conference, reporting what was being said sotto voce by the delegates.
The Guardian Dec 3, 2017

(source: vocabulary.com)