Which term refers to playing in a low, quiet voice?

The term that refers to playing in a low, quiet voice is "piano." This Italian term specifically indicates a soft dynamic level, guiding the performer to play gently. It is often abbreviated as "p" in musical scores and is fundamental in conveying the nuances of volume in music.

In contrast, "mezzo" translates to "medium," indicating a moderate volume, while "fortissimo" denotes playing very loudly. "Sotto voce," which means "under the voice" in Italian, implies a subdued or whispered manner of playing or singing, but it does not specifically refer to the low volume on its own. Thus, "piano" is the most accurate term for a low, quiet sound in music performance.