Brillouin light scattering (BLS) has been used to investigate the elastic properties of polycrystalline AlN films, about 1 mu m thick, grown by DC-reactive magnetron sputtering on Si3N2 coated (100)-Si substrates. Taking advantage from the detection of a number of different acoustic modes. a complete elastic characterization of the films has been achieved. The elastic constants c(11) and c(66) have been selectively determined from detection of the longitudinal and of the shear horizontal bulk modes, respectively, travelling parallel to the film surface. The three remaining elastic constants, namely c(44), c(33) and c(13), have been obtained from detection of the Rayleigh surface wave and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AlN films depend on the deposition conditions and on the microstructural properties of the films, especially oxygen contamination and quality of texture. In the case of the films with the best degree of texture and the lowest oxygen content, the values of the elastic constants an rather close to those previously determined in epitaxial AlN films grown at high temperature by MOCVD. This demonstrates that sputter deposition at relatively low temperature can be used to grow high quality AlN films on Si and is of great importance in view of the integration of these films in the technology of IC semiconductors.