The transient two-fluid model has been used to develop a general relation for acoustic waves with steam-water two-phase mixture in one-dimensional flowing system. The analysis is valid in principle over the whole void fraction region. Both the mechanical and thermal nonequilibrium are considered. The vapor-liquid interface heat flux is derived from the one dimensional Fourier heat conduction equation to evaluate the interphase evaporation or condensation rate. Calculations are performed for pressures from 0.07 to 16.0MPa, void fractions from 0.0 to 1.0. Higher frequency limit of sonic velocities are only relied on the pressures and void fractions, and are insensitive to the bubble radius, tube diameter, and the velocity difference between the two phases. The predicted sonic velocities are compared with some experimental data for low pressures. Good agreement has been achieved between the predictions and the experimental data.