Long-wavelength acoustic phonons have been studied for each of the $[00\ensuremath{\zeta}]T$, $[00\ensuremath{\zeta}]L$, $[0\ensuremath{\zeta}\ensuremath{\zeta}]{T}_{1}$, and $[0\ensuremath{\zeta}\ensuremath{\zeta}]L$ branches in KBr at 95 and 463\ifmmode^\circ\else\textdegree\fi{}K by means of inelastic neutron scattering. This study has shown that the elastic constants of KBr determined by inelastic neutron scattering (zero-sound regime) exhibit a different temperature dependence from those determined by ultrasonic techniques (first-sound regime). For example, the change in the zero-sound elastic constant for the $[0\ensuremath{\zeta}\ensuremath{\zeta}]{T}_{1}$ branch between 95 and 463\ifmmode^\circ\else\textdegree\fi{}K is (23\ifmmode\pm\else\textpm\fi{}4)%, while the change in the first-sound elastic constant determined by Hauss\"uhl using ultrasonic techniques is (35.1\ifmmode\pm\else\textpm\fi{}1.2)%. Cowley's theory for first and zero sound shows reasonable agreement with experiment.