  The anisotropic, stress-dependent velocity of elastic waves in glass beads --
as observed by Y. Khidas and X. Jia, see [Phys. Rev. E, 81:021303, Feb. 2010]
-- is shown to be well accounted for by ``granular solid hydrodynamics,'' a
broad-range macroscopic theory of granular behavior. As the theory makes no
reference to fabric anisotropy, the influence of which on sound is in doubt.
