  We study the dynamics of dilute and ultracold bosonic gases in a quasi
two-dimensional (2D) configuration and in the collisionless regime. We adopt
the 2D Landau-Vlasov equation to describe a three-dimensional gas under very
strong harmonic confinement along one direction. We use this effective equation
to investigate the speed of sound in quasi 2D bosonic gases, i.e. the sound
propagation around a Bose-Einstein distribution in collisionless 2D gases. We
derive coupled algebraic equations for the real and imaginary parts of the
sound velocity, which are then solved taking also into account the equation of
state of the 2D bosonic system. Above the Berezinskii-Kosterlitz-Thouless
critical temperature we find that there is rapid growth of the imaginary
component of the sound velocity which implies a strong Landau damping. Quite
remarkably, our theoretical results are in good agreement with very recent
experimental data obtained with a uniform 2D Bose gas of $^{87}$Rb atoms.
