In which context would a musician use 'sons bouches'?

The term 'sordino' or 'sons bouches' refers to a technique used in music where the sound is muted. This is achieved by using a mute for an instrument, or by employing a specific technique on instruments such as strings or brass to dampen the sound. This technique reduces the volume and alters the timbre, creating a softer and often more nuanced sound.

In orchestral and chamber music, using a muted effect can evoke different emotions and colors within the performance, which is why musicians might utilize this instruction during particular passages to contribute to the overall interpretation of the piece. This concept is foundational in understanding various expressive techniques used in music, as it allows performers to explore the subtleties of sound production.

The other contexts presented in the options pertain to different musical directions: a crescendo suggests an increase in dynamics, rapid octave shifts involve significant leaps between pitches, and legato indicates a smooth, connected style of playing. Each of these serves different expressive purposes but does not relate to the concept of muting sound.