Ah, I think I see what you mean.

I think the right way to think about this is as a wave, and in that respect, it's no different from sound waves. If sound goes through the air, then through another medium, then out the other side back to air, the speed will follow the same general rules as with light. One notable differece is that speed of sound through many materials is faster than through air.

Note also that you'll get acoustic reflections at interfaces, which is analogous to optical (Fresnel) reflections at interfaces (e.g., reflection off of glass or water).