The explosion at Helms Deep did kind of underwelhm (??) me, at first. Then I thought about it...

I think it's more of a subtle explosion for dramatic effect. As in, until that explosion, the good guys (elves, humans, dwarf, wizard,etc.) are basically doing well enough in the battle for one to think they have a chance of winning easily. Then the explosion takes place and the tone of the entire battle changes. The lack of sound to the explosion helps show this change of momentum. Now, the bad guys (orcs, goblins, etc.) seem to have the advantage. Then the music starts again to pick up the pace which lasts until Gandalf shows up to save the day.

At least, that's my .02 cents on that. May have to listen to some of the commentary tracks for some other clues...