A common feature in the density of vibrational states of glasses and supercooled liquids is the presence of an excess of modes over the Debye level known as boson peak. Despite the long-standing interest in this glass-specific excess of modes, its origin still remains poorly understood. We present new high quality experimental data obtained by means of the inelastic x-rays and neutron scattering techniques on a set of glassy systems characterized by different fragility parameters. The studied systems present well defined acoustic-like excitations at the lower accessible wavelengths. On increasing the frequency the excitations get rapidly overdamped and the apparent sound velocity presents a peculiar negative dispersion effect. This reduction of the sound velocity and the fast growth of the modes damping can be directly linked to the excess of modes at the boson peak. We will present a detailed analysis of these new findings and a comparison with a recently proposed model for the vibrations in glasses, which treats the glass as a continuum medium with the shear modulus exhibiting random spatial variations.