"There are a lot of people without jobs right now. It's not their fault," Alexis whispered, trying to model an appropriate indoor voice for her father.
2014, Bobbi Govanus, How To Pilot When We Were Raised to BE Stewardesses: Reinventing Your Life with Passion and Purpose, Lulu.com (2014), →ISBN, page 18:
She knew when I was on a business call that she had to be quiet and use her indoor voice.
She's easy to be with, provided you don't mind the constant chatter, the non-sequitur song cues, or her total lack of an indoor voice.
2015, Mary Ellen Guffey & Dana Loewy, Essentials of Business Communication, Cenage Learning (2015) →ISBN, page 376:
Practice smartphone etiquette by being considerate, observing quiet areas, using your indoor voice, taking only urgent calls, not calling or texting while driving, and choosing a professional ringtone.
2015, Denise Hunter, Married 'til Monday, Thomas Nelson (2015), →ISBN, page 46:
The air conditioner woke her each time it kicked on, and a kid next door didn't seem to have an indoor voice—or a bedtime.