What dynamic marking indicates a sound that is soft?

The dynamic marking that indicates a sound that is soft is "piano." In musical notation, "piano," which is often abbreviated as "p," directs the musician to play softly. This term comes from the Italian word for "soft" and is a foundational concept in dynamics, dictating the volume at which music is performed.

To enhance the understanding, "pianissimo" refers to an even softer sound than piano, indicating that the music should be played very softly, but it is the term "piano" itself that directly signifies the baseline for soft playing. "Forte," on the other hand, instructs musicians to play loudly, while "mezzo forte" suggests a moderate loudness—neither of which pertains to soft dynamics. Therefore, piano is the correct choice to signify soft sound in music.