We discuss the properties of waves in a mixture of a diffuse phase (phase I) and a dense phase (phase II) by linear analysis, considering the effects of phase transitions. Each phase has a different density and temperature, but pressure balance is assumed between them in equilibrium state. This condition, for example, corresponds to a two-phase model of the interstellar medium. Previously, we have shown that two distinct wave modes coexist in a two-phase mixture with frozen phase composition: one mode corresponds to a sound wave and the other mode to a void wave. Void waves are known to be fundamental waves in multiphase flows. Moreover, it has been also shown that evaporation of cold dense components results in hydrodynamic instability of flows in the surrounding hot gas when the speed of the flow dominates the sound speed of the hot gas.In this paper, we perform linear analysis to examine the effects of phase transitions on waves in a two-phase medium. We show that void waves are damped because of a ``pseudodrag'' force that arises because of mass and momentum exchange between hot and cold phases. Acoustic waves are shown to be unstable when mass exchange between phases increases the pressure of the volume-dominating phase.