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."