Dubbing sound in post production is called the art of Foley. > Why not just using the real rain sound? Simple sounds such as rain already exist in vast filmmaking sound effects libraries so there's really no need to duplicate those. read more
Though it's a great sound effect, its use does typically pull me completely out of the scene it appears in, and as such it's best used on silly action movies rather than, say, a serious piece of work like The Dark Knight Rises or something. read more
Foley artists are the movie personnel responsible for those and most sounds you hear in films. If filmmakers wait to record real rain sounds, they would need no other noises present and valuable man hours would be lost. read more