It´s not just sound, it´s the fire in space aswell. But just like a fire, sound can have the exacty same explanation:
Sound is nothing more than reverberations on particles of any given matter. Like water, gas, stone, etc. Each has it´s own tone and level of "noise". So, maybe we can´t hear anything in the middle of void empty space, but when we have a ship exploding, there is a whole lot of matter being jettisoned to space. Just like fire consumes the oxygen within the atmosphere held inside the ship, so do sound waves get to reverberate on that same flash of matter blasted on all directions. Technically, you SHOULD be able to hear them at a given distance, for the ammount of time those particles take to scatter into the vacuum of space.
When a missile explodes in space, assuming the warhead carries within itself the matter needed to provoke an explosion, it also spreads bits and pieces of matter of the missile itself. As long as the pilot keeps within the blast radius where the sound can travel to him, there should be no problem hearing it blow up. It wouldn´t sound as back here on Earth, but we would definitely hear something.
Let´s not forget that space isn´t all void, we have nebulae, asteroid fields, a whole bunch of places littered with particles of matter that can reverberate to produce sound.