We summarize the properties of waves in a mixture of a diffuse phase (phase I [P I]) and a dense phase (phase II [P II]) by linear analysis, introducing a method of multiphase fluid dynamics (MPFD). Each phase has different density and temperature, but pressure balance is assumed among them. This condition corresponds to the two-phase model of interstellar matter.First, we show that two distinct modes coexist in a two-phase mixture. One mode (the f-mode) corresponds to a sound wave, and the other mode (the s-mode) to a void wave. Void waves are known to be fundamental waves in multiphase flows. In particular, our s-mode appears when the P I medium compressed by blobs (P II) repels the blobs. According to a recent work by the present author, through this s-mode of void waves, it is suggested that small-scale structures appear in a self-gravitating two-phase gas mixture. Next, we perform numerical analysis and summarize the properties of both modes. In particular, in the s-mode, the phase velocity is not varied even if the γI (adiabatic exponent of P I gas) has various values. However, for large α, the phase velocity of the s-mode is reduced. Here α denotes the volume fraction of phase I. Thus, we find not only that the s-mode is insensitive to the intrinsic compressibility of the ambient gas around the blobs (γI effect) but also that the amount of P I medium compressed by ballistic motions of P II (the α effect) is effective in propagation of the s-mode in a two-phase interstellar gas mixture.