  We study the vibrational spectra and the specific heat of disordered systems
using an effective hydrodynamic framework. We consider the contribution of
diffusive modes, i.e. the 'diffusons', to the density of states and the
specific heat. We prove analytically that these new modes provide a constant
term to the vibrational density of states $g(\omega)$. This contribution is
dominant at low frequencies, with respect to the Debye propagating modes. We
compare our results with numerical simulations data and random matrix theory.
Finally, we compute the specific heat and we show the existence of a linear in
$T$ scaling $C(T) \sim c\,T$ at low temperatures due to the diffusive modes. We
analytically derive the coefficient $c$ in terms of the diffusion constant $D$
of the quasi-localized modes and we obtain perfect agreement with numerical
data. The linear in $T$ behavior in the specific heat is stronger the more
localized the modes, and crosses over to a $T^{3}$ (Debye) regime at a
temperature $T^{*}\sim \sqrt{v^{3}/D}$, where $v$ is the speed of sound. Our
results suggest that the anomalous properties of glasses and disordered systems
can be understood effectively within a hydrodynamic approach which accounts for
diffusive quasi-localized modes generated via disorder-induced scattering.
