  Inspired by the recent experimental signatures of relativistic hydrodynamics
in graphene, we investigate theoretically the behavior of hydrodynamic sound
modes in such quasi-relativistic fluids near charge neutrality, within linear
response. Locally driving an electron fluid at a resonant frequency to such a
sound mode can lead to large increases in the electrical response at the edges
of the sample, a signature which cannot be explained using diffusive models of
transport. We discuss the robustness of this signal to various effects,
including electron-acoustic phonon coupling, disorder, and long-range Coulomb
interactions. These long range interactions convert the sound mode into a
collective plasmonic mode at low frequencies unless the fluid is charge
neutral. At the smallest frequencies, the response in a disordered fluid is
quantitatively what is predicted by a "momentum relaxation time" approximation.
However, this approximation fails at higher frequencies (which can be
parametrically small), where the classical localization of sound waves cannot
be neglected. Experimental observation of such resonances is a clear signature
of relativistic hydrodynamics, and provides an upper bound on the viscosity of
the electron-hole plasma.
