  Using a simple thermo-hydrodynamic model that respects relativistic
causality, we revisit the analysis of qualitative features of acoustic
oscillations in the photon-baryon fluid. The growing photon mean free path
introduces transient effects that can be modelled by the causal generalization
of relativistic Navier-Stokes-Fourier theory. Causal thermodynamics provides a
more satisfactory hydrodynamic approximation to kinetic theory than the
quasi-stationary (and non-causal) approximations arising from standard
thermodynamics or from expanding the photon distribution to first order in the
Thomson scattering time. The causal approach introduces small corrections to
the dispersion relation obtained in quasi-stationary treatments. A dissipative
contribution to the speed of sound slightly increases the frequency of the
oscillations. The diffusion damping scale is slightly increased by the causal
corrections. Thus quasi-stationary approximations tend to over-estimate the
spacing and under-estimate the damping of acoustic peaks. In our simple model,
the fractional corrections at decoupling are $\gtrsim 10^{-3}$.
