The speed of propagation U of an infinitesimal one-dimensional disturbance of particulate phase pressure in dry granular flow is analyzed. We regard the propagation of such a discontinuity as the analog of the speed of sound in the classical continuum theory. Indeed, we show that, on the basis of the pseudo-thermodynamic theory of granular flow which we have recently presented, U is the square root of the inverse iso-pseudo-entropic compressibility of the particulate phase. Direct calculation of the value of U is discussed.