  Inhomogeneities associated with the cosmological QCD and electroweak phase
transitions produce hydrodynamical perturbations, longitudinal sounds and
rotations. It has been demonstrated by Hindmarsh et al. that the sounds produce
gravity waves (GW) well after the phase transition is over. We further argue
that, under certain conditions, an inverse acoustic cascade may occur and move
sound perturbations from the (UV) momentum scale at which the sound is
originally produced to much smaller (IR) momenta. The weak turbulence regime of
this cascade is studied via the Boltzmann equation, possessing stationary power
and time-dependent self-similar solutions. We suggest certain indices for the
strong turbulence regime as well, into which the cascade eventually proceeds.
Finally, we point out that two on-shell sound waves can produce one on-shell
gravity wave, and we evaluate the rate of the process using a standard sound
loop diagram.
