The STAR correlations data was used to to determine the sound at the last moment of the collision and the acoustic horizon tells us how that sound must have developed with time. Just like the horizon is the limit of how far we can see, the acoustic horizon limits the wavelengths that we can hear: Initially, only the shortest of wavelengths can fit inside the horizon, so we hear the higher tones first. As time passes the horizon expands. Once half a wavelength of a note fits inside of the horizon, the new longer wavelengths and lower note becomes audible. Finally, as the system becomes too cool and too spread out to interact anymore, we hear the final sound before the silence of vacuum.

Power Spectrum
Speed of Sound
Acoustic Horizon
The Sound