  Hard partons propagating through hot and dense matter loose energy, leading
to the observed depletion of hard hadron spectra in nucleus nucleus collision
as compared to scaled proton proton collisions. This lost energy has to
reappear in the medium due to the conservation of energy. Apart from heating
the medium, there is the possibility that a propagating collective mode is
excited. We outline a formalism that can be used to track the propagation of
such a mode through the evolving medium if its dispersion relation is known and
to calculate the resulting distortion of hadronic spectra at freeze-out. Using
this formalism, we demonstrate within a detailed picture of the evolution of
the expanding system and the experimental trigger conditions that the
assumption of a sound mode being excited is in line with 2-particle correlation
measurements by PHENIX and STAR for a semi-hard trigger. In this case, the
measurement is sensitive to the averaged speed of sound in the hot matter. We
also make suggestions how this sensitivity can be improved.
