It is shown that the presence of the sound waves causes the ionization balance to be altered from the quiescent plasma condition and the effect on excitation is also considered. The treatment covers isothermal and adiabatic sound waves in three plasma conditions, viz., (i) Uniform high density plasma in local thermodynamic equilibrium (LTE), (ii) Uniform low density plasma (non-LTE), (iii) Low density plasma of uniform pressure and a temperature gradient. The results show that the presence of a sound wave can cause quite large changes in the ionization balance depending on the magnitude and frequency of the waves as well as the atomic parameters. The intensities are modulated with the same frequency as the waves and the time or space averaged intensities can also be significantly modified. In the case with the temperature gradient the counter-acting effects of ionization balance and excitation are shown to reduce the extent to which the time averaged intensities are changed. The theory has been developed because of its possible application to the interpretation of the solar spectrum arising from the transition region between the chromosphere and corona where one of the proposed mechanisms for heating the corona requires propagating sound waves.