  In superfluid systems several sound modes can be excited, as for example
first and second sound in liquid helium. Here, we excite propagating and
standing waves in a uniform two-dimensional Bose gas and we characterize the
propagation of sound in both the superfluid and normal regime. In the
superfluid phase, the measured speed of sound is well described by a two-fluid
hydrodynamic model, and the weak damping rate is well explained by the
scattering with thermal excitations. In the normal phase the sound becomes
strongly damped due to a departure from hydrodynamic behavior.
