A general theory is presented for sound attenuation and dispersion near the critical point in the region of frequencies which are much greater than the characteristic frequency of order-parameter dynamics, but are much smaller than the frequency of sound with the wavelength equal to the range of correlation of local order-parameter fluctuations. The method used is the most general version of the mode-mode coupling theory plus the static and dynamic scaling hypotheses and some general properties of time-correlation functions. If we ignore the small critical exponent $\alpha$ associated with the heat-capacity singularity, we find that the attenuation behaves as $f{\epsilon }^0$ and the dispersion (relative sound-velocity change with frequency) as $f^0{\epsilon }^0$, where $f$ is the dimensionless frequency and $\epsilon$ the dimensionless temperature distance from the critical point.

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