  We theoretically study the sound propagation in a two-dimensional weakly
interacting uniform Bose gas. Using the classical fields approximation we
analyze in detail the properties of density waves generated both in a weak and
strong perturbation regimes. While in the former case density excitations can
be described in terms of hydrodynamic or collisionless sound, the strong
disturbance of the system results in a qualitatively different response. We
identify observed structures as quasisolitons and uncover their internal
complexity for strong perturbation case. For this regime quasisolitons break
into vortex pairs as time progresses, eventually reaching an equilibrium state.
We find this state, characterized by only fluctuating in time averaged number
of pairs of opposite charge vortices and by appearance of a quasi-long-range
order, as the Berezinskii-Kosterlitz-Thouless (BKT) phase.
