In most cases the energy dissipated in plasma jets used either for heating or spraying is varied by changing the are current, the total gas flowrate, and composition. However, when doing so, conditions are reached where tire plasma jet may become supersonic. To predict such conditions or to characterize supersonic plasma jets the knowledge of the sound velocity a is mandatory. The goal of this paper is to calculate a ver sus plasma-forming gas composition, temperature, and pressure. Rigorous calculation would imply the knowledge of the chemical reaction kinetics, sound velocity depending strongly on this. Unfortunately such kinetics are generally unknown for plasma jet flows and the only possibility is to determine the equilibrium sound velocity a calculated through the isentropic coefficient Gamma. This coefficient has been calculated taking into account the dissociation and ionization reactions at equilibrium for temperatures ranging from 300 to 25,000 K and pressures between 0.1 and 1 MPa for Ar, H-2, He, Ar-He, Ar-H-2, O-2, N-2, air, steam, and methane. a(gamma), often called the "frozen" sound velocity, was also calculated using gamma (ratio of specific heats) instead of Gamma.