"Ultimately it increases entropy... let me tell you about the heat death of the universe..."

"No, Mom! I'm still afraid of the False Vacuum monster laying underneath my ground state!"

Heat. Everything ends up as heat.

I imagine a physicist would invoke entropy to describe the diffusion of pressure waves and vibration into other forms of energy. Neuroscience might explain the propagation of signals from the cochlea into the brain. A psychologist could hypothesise on the influence of music on our mood and ideas. A philosopher might talk about the influence of music on the way we build our society and how that feeds back into our music. In this way, the music never stops, it continues on as echos rippling through through the universe.

You absorb it into your soul and it changes you irreversibly forever.

As the waves from ocean, the music crashes on the beaches in your ears

Everyone knows this duh!

Into your heart!

It gets stuck in my head for 3 to 4 days until something newer comes along and pushes it out.

Where does the light go when you flip the switch?

Check your fridge.

It rolls up into the other side of the cassette. Just flip it over.

I'm not a scientist by a long shot, but my understanding is that sound if indeed a wave, carried by a medium (air, water, etc). Upon hitting your eardrum, this wave is converted by your eardrum and your auditory nerve into signals your brain decodes. The remainder of the wave continues though, until it runs out of medium, hits an obstacle (basically another medium) or dissipates. Again, just my layman's understanding!

Sounds move very very fast, faster than a runner or a car, so it goes very very far away.

It goes into your memory. That's why you can remember a song that you heard before.

The Langoliers eat it.

It stays in our brain and we subconsciously put it into new music years later, thereby keeping the industry’s corporate lawyers in cocaine for future decades to come.

It keeps traveling. If you splash some water, where does the wave go? Same question - it terms into something you can no longer see or hear... It never goes away. It becomes part of the world, forever

Music is what you hear - but it was only ever sound waves

Oh, dear child, it goes to the same place where you will go when you inevitably die one day: into complete non-existence, save for an echo in others' minds, and after a while not even that.

Sweet dreams!

The best and the worst go straight to your brain and live there rent free.

Unfortunately, nobody has figured out how toget rid of the bad songs that drown out the good ones.

The Langoliers eat it

Some of that sound rearranges some of your neurons so that you can listen to Never Gonna Give You Up whenever you read this

After you listen to a song, the secret police from the RIAA come and lock it up in a small, dank cell given minimal sustenance, until the next time they can send it to some seedy hotel, suburban home, or automobile, to turn a trick and make them some more money, like some sort of whoo-re for the ears.

the music goes back again to be later re streamed to other people that might need it

In your ears

It turns into memories and heat.

Tell him about the day that music died

Into darkness, my old friend.

I've come to talk with you again.

Because a vision softly creeping

Left its seeds while I was sleeping

And the vision that was planted in my brain Still remains

Within the sound of silence

"Everywhere, all at once. That's why if you put your ear close to speakers it might collect too much and that can hurt your ear"

It dissipates into the air like butterflies, or a cloud of dust.

Oh, and while the king was looking down

The jester stole his thorny crown

The courtroom was adjourned

No verdict was returned

And while Lenin read a book on Marx

A quartet practiced in the park

And we sang dirges in the dark

The day the music died

It's still there, it's just in the past now.

"Where do you think it goes?"

Let's assume the kid knows it's a recording. It's still a valid question.

Like where is the recording coming from when the kid asks Alexa to play a song?

I never thought about it, as I don't have kids, but must be a bit harder explaining a global IT-infrastructure than it was for my grandpa to explaining how a VHS works. On a generalised level, that is.

All these physics answers! The funnest explanation for a kid is just that music is only there when you're listening to it. If you don't listen, there's no music. When you start listening again, music comes back. Then ask if they can hear the music of the wind.