sot ‧ to vo ‧ ce /ˌsɒtəʊ ˈvəʊtʃi $ ˌsɑːtoʊ ˈvoʊ-/ BrE AmE adverb formal
[ Date: 1700-1800 ; Language: Italian ; Origin: 'under the voice' ]
in a very quiet voice, so that other people cannot easily hear:
‘No, it was Daniel,’ she continued, sotto voce.