Maxwell's equations show that the speed of light is invariant in all inertial reference frames.
In other words, the speed of light is the same no matter what speed you are going.
This was a bigger inspiration to Einstein than the MM experiments.
Without the experiment the equations are just some symbols that can but don’t have to have any connection to the reality.
Speed of sound is invariant in all intertial reference frames. Speed of sound of supersonic plane is constant. Speed of sound in supersonic plane is constant. Air does not exists: it's contraction and expanding of space-time, not an air. (Joke)