Question

Why are light and sound considered as matter

Answer 1

Light and sound do not have mass. Light is electromagnetic wave transverse in nature while sound is longitudinal wave but since it requires a material medium to travel it is a mechanical wave. 
Moreover matter is the one which occupies the space. But waves do not occupy space. Electromagnetic waves don't need material medium, but sound waves do need a material medium to travel. Hence light and sound aren't considered as matter.

Answer 2

Matter is one which occupies space.But light is a beam and sound is a wave, these won't occupy space .Which means they donot have any mass 
 So light and sound are not considered as matter.