Ithink of you daily, my darling, especially when it's dark and the other men in the foxhole become silent. i like to pretend that the gunfire is a fireworks display and that you are here with me to enjoy it. who is the intended audience for this passage?

The intended audience is the soldier's wife or girlfriend. 

It is clear that the author is currently in the middle of a war, based on the word "foxhole" (a trench that a soldier could hide in) and the fact that he is surrounded by gunfire. It is also clear that he is writing to his wife or girlfriend because he addresses the letter to "my darling."