Alright- So the bazooka and the spear meet, but based on the speed of the rocket and the speed of the thrown spear they will meet and detonate roughly 2/3 of the way towards the intended target. 

The sound of the explosion will not be as adverse to a person using the weapon, to wit men who use heavy weapons repeatedly in combat; the sound becomes an accustomed and normalized effect, especially outside where the sound can dissipate more readily than in a confined area. 

As Gado states there will be little in the way of a crater since the explosion occurred in the air, and the majority of the explosive force would be directed towards The Archivist, but will have minimal effects since it was not charged and had been blunted by a charged defensive move. 

While I am not sure how much of sound the spider will make scrambling across the sandy beach, probably minimal so it would require a highly acute sense of sound to hear the creature. However while cloaked the spider will leave a tell-tale sign of his movement across the sand. It is possible that tracks will be seen but at the speed the spider is moving towards the demon a normalized attack can occur or other defensive measures can be employed with some creative thought.