Hydrodynamics captures the universal low energy collective dynamics of interacting quantum field theories. In disordered metals, it predicts purely damped collective dispersion relations with undefined energies at low momenta. However, its range of applicability is limited in the absence of a hierarchy between the electron-electron and electron-impurity mean free paths. Through a careful comparison of hydrodynamics with microscopic models of metals, we establish that such dispersions occur in correlated quantum matter whenever the electronic fluid undergoes momentum relaxation due to explicit breaking of translation by impurities. As a specific example of these momentum-relaxed modes we consider the recent discovery of an acoustic plasmon - dubbed Pines' demon - in Sr$_2$RuO$_4$. The observed dispersion of this neutral mode differed significantly from the massless linear behaviour predicted by the random phase approximation. We demonstrate that the observed dispersion corresponds, in fact, to a momentum-gapped quasiparticle.