When we light a solid lump of explosive it burns slowly layer by layer, but when we light up powder form of explosive it just burns with a huge explosion and sound why does it happen. And one more question, what fraction of the energy in a blast is lost just in the form of sound?
Gunpowder is a low speed explosive (by strict modern standards it isn't really explosive)
A burning explosive isn't the same as detonating. The combustion (burning) is spread from one part of the explosive to the next by the flame on the surface. In a large piece of explosive the burning can only reach the center slowly as the explosive burns away. In a finely powdered explosive there is a lot of surface area for the flame to reach quickly.
In a detonating explosive the detonation is set off by a supersonic shockwave so the entire explosive reacts 'almost' instantly