  Velocity and absorption coefficient of the plane sound waves in classical
gases are obtained by solving the Boltzmann kinetic equation. This is done
within the linear response theory as a reaction of the single-particle
distribution function to a periodic external field. The nonperturbative
dispersion equation is derived in the relaxation time approximation and solved
numerically. The obtained theoretical results demonstrate an universal
dependence of the sound velocity and scaled absorption coefficient on variable
$\omega\tau$, where $\omega$ is the sound frequency and $\tau^{-1}$ is the
particle collision frequency. In the region of $\omega\tau\sim 1$ a transition
the frequent- to rare-collision regimes takes place. The sound velocity
increases sharply, and the scaled absorption coefficient has a maximum -- both
theoretical findings are in agreement with the data.
