In many extensions of the Standard Model, the electroweak phase transition is first order. Such a phase transition proceeds via the formation and collision of bubbles. The bubble collisions can source a stochastic gravitational wave background signal, with characteristic frequency right in the sensitivity band of LISA. We can thus use data from gravitational wave experiments to probe physics beyond the standard model. In this talk, I will focus on the contribution to the gravitational wave signal from sound waves that get formed in the interactions between the plasma and the bubble walls. Predictions of the gravitational wave spectrum typically rely on hydrodynamic lattice simulations of the scalar-plasma system. Hydrodynamic solutions of a single expanding bubble provide a bridge between the particle physics model and the hydrodynamic lattice simulation. Two relevant quantities in this computation are the bubble expansion velocity and the kinetic energy budget. I will demonstrate how the speed of sound affects the kinetic energy budget, and discuss what values one expects for the speed of sound. I will then discuss the computation of the bubble wall velocity, and present a model-independent computation of the wall velocity, obtained in local thermal equilibrium.