Sound waves must have a material medium to propagate. They compress and decompress the medium. This compression requires motion of molecules that have inertia. It takes time to accelerate the molecules and create a compression wave. read more
Sound waves, being mechanical, must have a material substance to travel through, but electromagnetic waves can travel through a vacuum, where there is no matter as well as travel through material. Sound is, of course, the vibrations of air which we hear with our ears, but can be vibrations in solids like steel or liquids like water. read more
This is also why sound travels faster in a solid or liquid rather than in a gas as the particles are closer together in a solid, allowing the sound to travel much faster through it.Electromagnetic waves do not travel through this medium and travel at the speed of light (as thats what they are) when in a vacuum or in air. read more