wisteria darling
One that I watche which I think I will always remember was this young women who was supposedly possessed. After putting her in a trance and watching her flail and moan on the floor for a while, the woman started to speak in this deep unearthly voice.. the voice of the dead? She contiuned to answer the exorcist's questions (the spirit was now talking to him, actually, not the woman) and it stated that it was a princess who was thwarted by her lover who left her with a child.
Watch the Exorcism of Emily Rose, it explains how things like that work. If someone is having a certain kind of seizure one of the things they are capable of doing is activating both sets of vocal chords (human beings have two, its true) and it lets out a really messed up sound.