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Sound speed (speed of sound) is relevant to astrophysics despite the fact that "sound does not travel in a vacuum". Sound is compression waves in a gas (or a liquid or solid, or shear waves in a solid). "Space" is not a pure vacuum, and over an appropriate scale, the presence sound waves can sometimes be detected and can effects of interest. Sound speed is relevant to stars, e.g., asteroseismology, as well as to models of the mechanism creating stellar wind. It is also relevant to star formation, the process by which dense cores contract into protostars. And also to other cloud dynamics. It is also relevant to cosmology, e.g., baryon acoustic oscillations.
In some cases, the sound itself is relevant and in others, sound speed may be cited as a short-hand for the relevant factors of a gas, i.e., a gas's density, and bulk modulus (compressible elasticity). The same ratio of factors affects gas dynamics that may not be obviously related to sound.