It's worth pointing out that light doesn't always travel at the maximum speed of light, so there are occasions that sound will travel faster than light through something. I think that inside the Sun might even be one of these cases, as I've read somewhere that photons of light can take some thousands of years to go from the centre of the Sun to the outside (and then only minutes to get to the Earth from then).
In that case sound *does* travel faster than light in the sun. But that's nothing to do with information transfer, or cause and effect etc. Sound also travels faster than light through a solid wooden door, but information still reaches you no faster than the *in vacuum* speed of light.
So, in short, whether or not light travels slower inside the Sun than sound does, Sound is still stuck at slow speeds compared to the speed of light in vacuum. Hence causality isn't affected by this argument.