The gravitational shift of a photon in vacuum df/f=gH/c^2 had been confirmed by Pound and Rebca in 1960s. As to sound waves, the result is unclear. A proposal in 2000 was df/f=gH/(340m/s)^2 in air on the surface of the earth.( On the dynamics of mechanical waves and phonons, Journal of Pure and Applied Ultrasonics, volume 22, No.4, 106-110). On the other hand, the Mach angle sin=340/V (V is the velocity of motion) of supersonics was deduced from a new way (http://arxiv.org/abs/0911.2359 ) which implies the ratio between the energy and mass of a phonon is indeed the square of the speed of mechanical waves. This can be regarded as an indirect evidence to the above conclusion. Whether there exist other theories or not?