sotto voce

in a quiet voice, while speaking very softly

Sotto voce (literally "under voice"), an Italian expression, means to speak under one's breath or to speak confidentially. In music, a dramatic lowering of the vocal or instrumental tone -- not necessarily pianissimo, but with a hushed quality. May also denote muttering in theatrical or film scripts.
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Adverb
1. in an undertone; "he uttered a curse sotto voce"
(synonym) in a low voice

sotto voce, in a quiet voice, while speaking very softly

adv. sotto voce