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The part described by many that "propagation speed of light is always an absolute", IMO is wrong. Moving gun firing a bullet example, compared to moving flashlight are the same exactly if the light has not reached yet the remote observer (Galilean Relativity, light's $1st$ wavefront reaches sooner the observer when flashlight or observer is moving towards each other).

However, because macroscopically, light is a wave then when it already has reached the observer and has engulfed him/her/it, then yes, speed of light propagation is absolute and fixed and independent the observer's or source relative motions and only its frequency and wavelength changes relative to the observer's and source relative motion $c=λf$ (Special Relativity) (Doppler Effect).

But then, how this is different from sound waves? Does the propagation speed of sound changes by the relative motions of the source or observer in the case the observer is already inside and engulfed by these sound waves? The answer is again NO, propagation speed of sound inside the medium remains fixed and only the frequency and wavelength of sound observed changes with the motion of the observer relative to the source.

Personally, IMO, SR theory is actually describing here the Doppler effect of light waves.

Very few even professionals seem to understand this or forget to mention that special relativity theory explaining the moving flashlight relative to an observer "paradox", concerns only continuous plane waves case where the observer is assumed to be already and always inside the light waves.

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    $\begingroup$ That is not what SR describes. Have you read Relativity? Galilean relativity is quite different from Einsteinian/Lorentzian relativity. $\endgroup$
    – controlgroup
    Commented Dec 10 at 13:01
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    $\begingroup$ The speed of sound relative to an observer is independent of the motion of the source, but is dependent on the motion relative to the observer of the medium carrying the sound. There is no such dependency for light (for which the notion of a medium has no explanatory value). Have you read about the Michelson-Morley experiment for light? $\endgroup$
    – Philip Wood
    Commented Dec 10 at 13:08
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    $\begingroup$ Moreover, what’s the question here? Surely someone with your rep would know what does/doesn’t constitute an on-topic post here. If your question is on a sound-light analogy, maybe drop all the other stuff criticizing “most professionals”, some of whom will likely be the ones who review your question and might otherwise answer. $\endgroup$
    – controlgroup
    Commented Dec 10 at 13:13
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    $\begingroup$ how do you explain time dilation then? $\endgroup$
    – Pato Galmarini
    Commented Dec 10 at 13:28
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    $\begingroup$ Do you understand that "constant speed of light" is not a result of the theory of relativity? It's an axiom. The entire point of the theory of relativity was to show what the laws of physics would be like *IF* the speed of light was a universal constant. $\endgroup$
    – Solomon Slow
    Commented Dec 10 at 14:19

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