Sotto voce
With a muted voice.
What it means
Sotto voce means “under the voice” and calls for a muted, restrained performance. What is meant is more than mere softness: the sound is to seem held back, as it were spoken aside, as though it were not really meant to be heard. The instruction applies to singers as well as to instrumentalists and describes an attitude, not only a level of volume. It can stand together with a dynamic marking but does not replace it.
In the notation
The word stands in italics above the staff and applies until a new marking. It often appears together with piano or pianissimo, occasionally also at a medium dynamic, when the character rather than the volume is meant. It is cancelled by a contrary instruction or by a distinct change in the texture.
Musical use
In opera the instruction marks speaking aside, conspiracy, secrecy; in choral writing it creates a restrained surface out of which individual voices can emerge. Instrumentally it calls for a covered sound: string players pull back arm weight and play rather in the middle of the bow, wind players work with a calmer airstream. Its effect rests on contrast — it is worthwhile only if the surroundings sound different. It is therefore a means of organization as much as of expression.
Performance practice
Keep the tone supported despite the low volume; a breathy, unsupported sound does not carry and becomes impure. Watch for consonants that are all the more distinct, because otherwise the text is lost. In choir, uniformity is decisive: agree on the tone colour, not only the volume. In the orchestra, check the balance from the hall, because quiet passages work differently on the podium. Plan the transition into and out of this colour deliberately, so that the change is audible as intentional.
Bear in mind that the instruction means an attitude and not only a level of volume: it can also stand at a medium dynamic, when the character of speaking aside is meant. Check the context. Watch for consonants that are all the more distinct. Instrumentalists likewise use the marking, in the sense of a muted, held-back tone.