If lightning strikes in your backyard you will hear an immediate and very loud clap of thunder. If there is NO rain it is easy to also hear the very low pitched thunder rolling smoothly away off into the distance.

A~~~~~~~~~B~~~~~~~~~C~~~~~~~~~D~~~~~

If you are at "A" and the lightning strikes and claps at "A" it starts an expanding shock wave in the air that expands away spherically.
The shock-wave is so powerful it disrupts the atmosphere and what you think you hear as thunder rolling away is actually the shock wave being reflected back.
The shock-wave traveled from A to B in one second and it keeps going.
But if you were standing at any point along the path for instance "B" you would still hear the initial strike and clap as it passed by you at the speed of sound..
So what you are think you are hearing as thunder is actually the instantaneous strike and clap happening over and over and over.
It is continuous but not being stretched.
You are hearing one instant thing but it is continuous
The sound of course is being Doppler shifted -- it is racing away at the speed of sound.
If you have good ears and think you hear the thunder at "B" it is already at "C".
In the time it took the sound from "B" to travel back to you at "A" the clap has already moved on to "C".

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Big Bang / CMB
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When the Big Bang happened it disrupted the fabric of space itself in the same way a one time thunder clap continuously disrupts the atmosphere.
When you detect the CMB you are actually detecting the "thunder" of the Big Bang. But It is happening as electromagnetic radiation of course.
The CMB is actually the Big Bang happening over and over and over but it is red-shifted down to microwave spectrum.