A theory is developed for the sound velocity and attenuation in a medium composed of closely packed solid particles immersed in a fluid. The absorption mechanism considered is that of viscous motion of fluid between the particles, and the macroscopic point of view is taken. For high enough frequencies the attenuation is proportional to the square root of the product of frequency and static flow resistance of the medium. Comparisons are made with data reported previously by others, and the agreement with the theoretical results is good for cases involving particles of nearly uniform size. Still another frequency effect, dependent on the size distribution, is evident for unsorted granular substance.