Which of the following terms refers to a musical passage played softly or subdued?

The term that refers to a musical passage played softly or subdued is "sotto voce." This Italian phrase literally translates to "under voice," and it indicates that the music should be performed in a quiet or subdued manner, often conveying a sense of intimacy or secrecy.

In orchestral and vocal contexts, using "sotto voce" instructs musicians to lower their volume while maintaining clarity and expression in their performance. This technique can create a poignant atmosphere and draw listeners in closer to the music.

Understanding this term enhances musicians' interpretative skills and helps convey the emotional nuances intended by the composer. Other options relate to different musical concepts but do not specifically denote subdued dynamics in the same way. For instance, "sonore" generally refers to a rich, full tone, "sostenuto" implies a sustained or held sound, and "sempre" means "always" and does not convey any specific dynamic instruction.