Shock Hugoniots of metal with different initial volume and pressure have been calculated with the use of multi-phase equation of state. An application of D’yakov-Kontorovich’s criterion revealed an existence of specific sound instability in the liquid-gas region. The instability arises as a spontaneous sound emission from the shock discontinuity. The position of the instability region and its dependence on initial pressure and volume has been analyzed. Here general regularities obtained on the base of the analysis for 30 metals are discussed. ©2000 American Institute of Physics.