A Dictionary of Music and Musicians/Sotto Voce
SOTTO VOCE, 'under the voice,' in an undertone; a direction of frequent occurrence in vocal music, where its meaning is obvious. It is transferred however to instrumental music, where its meaning is less clear. By some performers it is considered that the diminution in tone should be produced by artificial means, as by the soft pedal on the piano, or the sordino on the strings, while others take it as simply equivalent to a kind of pp. It may be taken as a universal rule that a sort of hushed effect is intended. A notable instance of its use occurs in the opening of the Choral Symphony.
[ J. A. F. M. ]