  The speed of sound in QCD matter at finite temperature and density is
investigated within the Polyakov loop improved Nambu--Jona-Lasinio (PNJL)
model. The spinodal structure associated with the chiral first-order chiral
phase transition is considered to describe the continuous variation of the
speed of sound. The behaviors of the squared sound speed in different phases,
including the stable, metastable and unstable phases, are derived. The relation
between speed of sound and QCD phase transitions is systematically explored. In
particular, the boundary of vanishing sound velocity is derived in the
temperature-density phase diagram, and the region where the sound wave equation
being broken is pointed out. Some interesting features of speed of sound under
different definitions are also discussed.
