  We analyze longitudinal pion spectra from E_lab= 2AGeV to \sqrt s_NN=200GeV
within Landau's hydrodynamical model. From the measured data on the widths of
the pion rapidity spectra, we extract the sound velocity c_s in the early stage
of the reactions. It is found that the sound velocity has a local minimum
(indicating a softest point in the equation of state, EoS) at E_beam=30AGeV.
This softening of the EoS is compatible with the assumption of the formation of
a mixed phase at the onset of deconfinement.
