High-pressure Brillouin scattering measurements have been carried out in orientationally disordered (OD) phases I and I' of solid HBr, which yielded the first determination of acoustic velocities for all directions, the refractive index, the polarizability, the density, adiabatic elastic constants (C-11, C-12, and C-44), bulk modulus, and the elastic anisotropy as a function of pressure up to 7 GPa and at room temperature. The I --> I' phase transition in solid HBr was confirmed on the discontinuous pressure dependence at P = 2.3 GPa for average errors of the least-squares fit by the analysis of in situ Brillouin spectroscopy. These elastic properties of solid HBr are compared with recent results in the OD phase I of solid HCl. We suggest that the form of the potential energy function for intermolecular forces in solid HBr is similar to that in HCl.