  We derive the low-energy effective action governing the infrared dynamics of
relativistic superfluids at finite temperature. We organize our derivation in
an effective field theory fashion-purely in terms of infrared degrees of
freedom and symmetries. Our degrees of freedom are the superfluid phase \psi,
and the comoving coordinates for the volume elements of the normal fluid
component. The presence of two sound modes follows straightforwardly from
Taylor-expanding the action at second order in small perturbations. We match
our description to more conventional hydrodynamical ones, thus linking the
functional form of our Lagrangian to the equation of state, which we assume as
an input. We re-derive in our language some standard properties of relativistic
superfluids in the high-temperature and low-temperature limits. As an
illustration of the efficiency of our methods, we compute the cross-section for
a sound wave (of either type) scattering off a superfluid vortex at
temperatures right beneath the critical one.
