  We theoretically investigate sound modes in a weakly-interacting collisional
Bose gas in $D$ dimensions. Using the Landau's two-fluid hydrodynamics and
working within the Bogoliubov theory, we observe the hybridization of the first
and second sound modes for $D\ge 2$. To model the recent measurements of the
sound velocities in 2D, obtained in the weakly-interacting regime and around
the Berezinskii-Kosterlitz-Thouless transition temperature, we derive a refined
calculation of the superfluid density, finding a fair agreement with the
experiment. In the 1D case, for which experimental results are currently
unavailable, we find no hybridization, triggering the necessity of future
investigations. Our analysis provides a systematic understanding of sound
propagation in a collisional weakly-interacting Bose gas in $D$ dimensions.
