  The two-fluid model is fundamental for the description of superfluidity. In
the nearly-incompressible-liquid regime, it successfully describes first and
second sound, corresponding, respectively, to density and entropy waves, in
both liquid helium and unitary Fermi gases. Here, we study the two sounds in
the opposite regime of a highly compressible fluid, using an ultracold $^{39}$K
Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode
of our homogeneous gas, and observe two distinct resonant oscillations below
the critical temperature, of which only one persists above it. In a microscopic
mode-structure analysis, we find agreement with the hydrodynamic theory, where
first and second sound involve density oscillations dominated by, respectively,
thermal and condensed atoms. Varying the interaction strength, we explore the
crossover from hydrodynamic to collisionless behavior in a normal gas.
