The single element without which music does not exist is sound. Sound travels through a medium, usualy air, but also through water or basicaly any gas or liquid. Can sound travel through a solid? I don't know, possibly.
But what is sound? Something you can hear you might say. This is true, but it differs from person to person. So what are the limmits of sound?
Wait, wait, what key is it in?
The most famous music in the wolrd?
That question can't be answered without further specification. The answer depends on what you see as famous.
The oldest music in the world?
Hurrian Hymn No. 6
c. 1400 BCE, Written on clay in cuneiform