Nitrogen-doped amorphous hydrogenated hard carbon films were deposited by r.f. glow discharge from methane-nitrogen mixtures onto silicon substrates. The elastic properties of films obtained for different N-2 partial pressures were investigated by Brillouin light scattering, while Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray reflectivity were exploited to study their structural and vibrational properties. Addition of a low percentage of nitrogen in the films induces marked modifications of the microscopic film structure, which induces a clear decrease of the elastic constants.