  We report on the first 3-dimensional numerical simulations of first-order
phase transitions in the early universe to include the cosmic fluid as well as
the scalar field order parameter. We calculate the gravitational wave (GW)
spectrum resulting from the nucleation, expansion and collision of bubbles of
the low-temperature phase, for phase transition strengths and bubble wall
velocities covering many cases of interest. We find that the compression waves
in the fluid continue to be a source of GWs long after the bubbles have merged,
a new effect not taken properly into account in previous modelling of the GW
source. For a wide range of models the main source of the GWs produced by a
phase transition is therefore the sound the bubbles make.
