  Recently interest in calculations of the speed of sound in QCD under
conditions like constant temperature $c^2_T$ or constant entropy per net baryon
number $c^2_s$ arose in the discussion of experimental results coming from
heavy ion experiments. It has been stressed that the former in particular is
closely related to higher order cumulants of conserved charge fluctuations that
are calculated in lattice QCD. We present here results on $c^2_T$ and $c^2_s$
and compare results at vanishing strangeness chemical potential and vanishing
net strangeness number with hadron resonance gas model calculations. We stress
the difference of both observables at low temperature arising from the light
meson sector, which does not contribute to $c^2_T$.
