Question

how do sound wave differ from mechanical wave?

Answer 1

Mechanical waves are a broad collection of waves of which sounds waves are a part.

The basic property of mechanical waves is that they can either be transverse or longitudinal but always require a medium to travel. They travel in such a way that particles don't really move from their initial positions but energy transfer does take place from one point to another.

So, they cannot exist in vacuum.

Finally, in short, sound waves are a type of longitudinal mechanical waves.