Question

why does sound travels with different speed in different medians

Answer 1

the closer the molecules of the medium, the tighter their bonds and sound travels faster. if the medium molecules are farther apart from one another, then sound travels slower in that medium. I.e. speed of sound depends on the force of attraction of the medium (solid, liquid or gas)

Answer 2

This is because :
in solids the molecules are tightly packed. So sound travels slowly through solid mediums.
In Liquids, the molecules are not as tightly packed as that in solids. So sound travels faster in liquids that in solids.
In Gases, the molecules are loosely packed, so sound travels the fastest in gas medium