[davrosthedalek]

The difference between sound and light is that sound needs a medium. This medium breaks the symmetry: There is a special system, the one in which the bulk of the medium doesn't move. For light, you don't have that. All reference systems are equal, independent of the speed they move with respect to each other. Light appears at the same speed in all of them. That's not possible with Newtonian velocity addition.

The experimental "proof" of the frame-independence comes from the michelson morley experiment. (Scare-quotes, because you can't prove things in physics, only disprove)