  Ultrarelativistic nuclear collisions create a strongly interacting state of
hot and dense quark-gluon matter that exhibits a remarkable collective flow
behavior with minimal viscous dissipation. To gain deeper insights into its
intrinsic nature and fundamental degrees of freedom, we determine the speed of
sound in an extended volume of quark-gluon plasma using lead-lead (PbPb)
collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data
were recorded by the CMS experiment at the CERN LHC and correspond to an
integrated luminosity of 0.607 nb$^{-1}$. The measurement is performed by
studying the multiplicity dependence of the average transverse momentum of
charged particles emitted in head-on PbPb collisions. Our findings reveal that
the speed of sound in this matter is nearly half the speed of light, with a
squared value of 0.241 $\pm$ 0.002 (stat) $\pm$ 0.016 (syst) in natural units.
The effective medium temperature, estimated using the mean transverse momentum,
is 219 $\pm$ 8 (syst) MeV. The measured squared speed of sound at this
temperature aligns precisely with predictions from lattice quantum
chromodynamic (QCD) calculations. This result provides a stringent constraint
on the equation of state of the created medium and direct evidence for a
deconfined QCD phase being attained in relativistic nuclear collisions.
