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.

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:-).

To retire on a farm upstate, unless my mom lied to me 🤔

It goes on until it doesn’t? Seems like there’s a pseudo-scientific philosophical argument that it continues on forever at some quantum level?

The music stays in your head until your forget it. Music only exists in your head

into our minds and into our hearts

It diverges into the quantum realm of misheard lyrics.

The music stays there we keep moving

But actually though, music goes up into the sky and becomes clouds.

Nothing's really ever gone

They should show their kid this.

As someone with a degree in Philosophy, I don't think this is really a philosophical question. The science is interesting and useful to know