  We analyze longitudinal pion spectra from \sqrt{s_{NN}}= 2 GeV to \sqrt
{s_{\rm NN}}=20 GeV within Landau's hydrodynamical model and the UrQMD hybrid
approach. From the measured data on the widths of the pion rapidity spectra, we
extract the sound velocity c_s^2 in the dense 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 \sqrt{s_{NN}}= 4-9 GeV, an energy range
accessible at the Facility for Antiproton and Ion Research (FAIR) as well as
the RHIC-Beam Energy Scan (RHIC-BES). This softening of the EoS is compatible
with the formation of a QGP at the onset of deconfinement. The extracted sound
velocities are then used to calculate an excitation function for the mean
transverse mass of pions from the hybrid model. We find that, above
\sqrt{s_{NN}} \approx 10 GeV, even the lowest c_s^{2} gives a considerably
larger <m_T> of pions compared to data.
