The acoustic wave mode in a weakly ionized gas, which is a perturbed version of an ordinary sound wave in a neutral gas, is investigated. At sufficiently low frequencies, ω<<ωc=Ωn(NnNi) (TnTe), the acoustic oscillations of the electrons, ions, and neutrals are all in phase and have equal amplitudes (Nn=neutral-particle density, Ni=ion-particle density, Tn=neutral temperature, Te=electron temperature, Ωn=collision frequency of a neutral particle with charged particles). However, at frequencies of the order of or larger than ωc, a marked phase difference between the oscillations of the different fluid components occurs. This leads to charge separation and an electro-acoustic effect, i.e., an electric-field perturbation produced by a sound wave. Previously reported wave amplification, predicted on the assumption that all particle species oscillate in phase, is found to be consistent with the dispersion relation derived here at frequencies ω<<ωc. At frequencies ω>=ωc, a reduction of the amplification takes place. As a consequence it is shown that, contrary to what previously has been believed, a decrease of the neutral-gas temperature does not always lead to an increase in the wave amplification.