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.

"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!"

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.

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

The Langoliers eat it

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

we hang it on a wall of silence in the room of our imagination

Everyone knows this duh!

Into your heart!

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

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!

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.

The Langoliers eat it.

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.

it goes into your soul

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

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!

"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"

Tell him about the day that music died

It goes out into the world, to be merged alongside all of the other sounds, until it can be recycled as "new" music and you can enjoy it again:-).

"Where do you think it goes?"