  The effects of different forms of the sound-velocity function c_s(T) on the
hydrodynamic evolution of matter created in the central region of
ultra-relativistic heavy-ion collisions are studied. At high temperatures
(above the critical temperature T_c) we use the sound velocity function
obtained from the recent lattice simulations of QCD, whereas at low
temperatures we use the ideal hadron gas model. At moderate temperatures
different interpolations between those two results are employed. They are
characterized by different values of the local maximum (at T = 0.4 T_c) and
local minimum (at T=T_c). The extreme values are chosen in such a way that at
high temperature all considered sound-velocity functions yield the entropy
density consistent with the lattice simulations of QCD. We find that the
presence of a distinct minimum of the sound velocity leads to a very long (~ 20
fm/c) evolution time of the system. Since such long evolution times are not
compatible with the recent estimates based on the HBT interferometry, we
conclude that the hydrodynamic description becomes adequate if the QCD
cross-over phase transition renders the smooth temperature variations of the
sound velocity, with a possible shallow minimum at T_c where the values of
c_s^2(T) remain well above 0.1.
