We have measured the resistivity and the ultrasonic sound velocity of La_0.7Ca_0.3MnO₃ as functions of temperature and magnetic field. As the temperature is lowered through T_c, where the magnetic phase transition occurs, the sound velocity increases rapidly in the narrow temperature region around T_c. The region where the velocity change occurs shifts up to higher temperatures, as a larger magnetic field is applied. The qualitative behavior of the sound velocity as a function of temperature may be explained in terms of phonon hardening which is due to the combined effect of the double-exchange and the strong electron-phonon interaction.