  Starting from our previous work where we have obtained a system of coupled
integro-differential equations for acoustic sound waves and phonon density
fluctuations in 2D crystals, we derive here the corresponding hydrodynamic
equations and study their consequences as function of temperature and
frequency. These phenomena encompass propagation and damping of acoustic sound
waves, diffusive heat conduction, second sound and Poiseuille heat flow, all of
which are characterized by specific transport coefficients. We calculate these
coefficients by means of correlation functions without using the concept of
relaxation time. Numerical calculations are performed as well in order to show
the temperature dependence of the transport coefficients and of the thermal
conductivity. As a consequence of thermal tension mechanical and thermal
phenomena are coupled. We calculate the dynamic susceptibilities for
displacement and temperature fluctuations and study their resonances. Due to
the thermo-mechanical coupling the thermal resonances such as Landau-Placzek
peak and second sound doublet appear in the displacement susceptibility and
conversely the acoustic sound wave doublet appears in the temperature
susceptibility, Our analytical results do not only apply to graphene but are
also valid for arbitrary 2D crystals with hexagonal symmetry like 2D h-BN,
2H-transition metal dichalcogenides and oxides.
