Sound travels at its slowest speed in a vacuum. Once it enters hotter mediums, it travels faster. On the other hand, light travels the fastest in a vacuum. Once it enters other mediums, it travels slower.
Why is one faster whereas the other slower?
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I suppose the answer you’re looking for is that light is a wave without a medium, whereas sound is not. The fact that light can propagate in vacuum at all was not originally obvious, and people thought it must need a medium (the “luminiferous aether”) because, after all, aren’t all waves an oscillation of some “stuff”?
Well, Maxwell’s Equations showed us that an EM wave is self-sustaining: the changing magnetic field begets a changing electric field, which begets a changing magnetic field, which...etc, and this happens in pure vacuum. Sure, light can travel through other stuff too, but other stuff just gets in the way, slows it down.
Sound, on the other hand, really is an oscillation of stuff. It’s not that sound is slow in vacuum, it simply does not exist. So stuff is necessary, and if you make that stuff rarer, then the sound wave might slow down, depending on the other mechanical properties.
