  In high energy nuclear collisions, QCD jets deposit a large fraction of their
energy into the produced matter. It has been proposed that as such matter
behaves as a liquid with a very small viscosity, a fraction of this energy goes
into a collective excitation called the ``conical flow'', similar e.g. to the
sonic booms generated by the supersonic planes. In this work we study the
effect of time-dependent speed of sound on the development of the conical wave.
We show that the expansion of matter and the decrease of $c_s$ leads to an
increase of observable manifestations of the conical flow. We also show that if
the QCD phase transition is of the first order (and thus with vanishing speed
of sound in the mixed phase) the wave must split into two, with opposite
directions. We then argue that it is not the case experimentally, which
supports the conclusion that the QCD phase transition is $not$ of the first
order.
