Question

why sound need medium to travel ?

Answer 1

Sound is a mechanical wave and needs medium for propagation. Sound travels through solids, liquids and gases. Sounds does not travel in vacuum.

Answer 2

in contrast,electromagnetic waves require no medium, but can still travel through one.since a vacuum defined as lacking  a medium,then sound and correspondingly,mechanical waves,cannot propagate through a vacuum .this is why we cant hear  voices and sound in space.