The propagation of sonic waves has been studied through thermally conducting and dissociating gases. The law of propagation is determined and it is found that the velocity of propagation relative to the normal gas velocity is the effective isothermal velocity of sound. The differential equations governing the growth and decay of sonic discontinuities are obtained and solved. The effects of dissociation, thermal conduction and that of the wave geometry on the global behaviour of the wave amplitude have been studied. It is found that there is an interesting competition between dissociation to resist the steepening tendency of a compressive weak wave to stabilize it and the thermal conduction effects to destabilize the wave. It is concluded that under dissociation effects, shock wave formation is either disallowed or delayed while thermal conduction accelerates the process of termination of a weak wave into a shock wave. Three cases, diverging waves, converging waves and plane waves have been studied separately with reference to the growth and decay behaviour of their amplitudes.