Me and the people I work with have a collection of EVP we have obtained. It is real. Some can be chalked up to static, background noise and such but we have a few that are undeniably real. Unless you decide that we are liars and invented them, so, I guess that's why these things are never proven. Anyway. Theory is basically what ladygriffen said. Due to the ghosts ability to effect electro magnetic field it can leave imprint on electronic recording devices.