Would it be more accurate to say that media other than "the vacuum" (Euclidean space less each ot its occupiers) causes light to take longer paths as opposed to losing velocity? Most material is mostly vacuum. Would not velocities of the source or detector simply change light's frequency not its velocity?

Could there be intellectual profit in hypothesizing a material (maybe dark matter) with a (most likely unobtainable modulus of elasticity) set of physical properties such that sound would travel through it at the velocity of light in a vacuum?

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?