1. In soft tones, so as not to be overheard; in an undertone: "There were aspersions cast, sotto voce, but knees quickly folded into curtsies when introductions were in order" (Barbara Lazear Ascher).
2. Music. In very soft tones. Used chiefly as a direction.

[Italian : sotto, under + voce, voice.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.