Seán Manchester (from his writings) on disembodied voices:
Disembodied voices can be attributed a variety of phenomena, and "ghosts" are one of them.
The basic Christian premise of good souls going to Heaven and the bad ones go to Hell is strongly questioned by a belief in ghosts. Ghosts are considered by most who believe in them to be the spirits of people who have never departed the Earth. These spirits travel around continuing to do the same things that were done in life. This is completely against the fundamental tenet of Heaven and Hell as understood by most Christians. Paradoxically, many Christians still claim to believe in the existence of ghosts.
The Catholic Church, however, believes that ghosts, ie spirits, do exist. There are even times when spirits appear to our benefit, but Catholics are warned against attempting to contact spirits.
“Ghost” is simply another word for “spirit” (geist means “spirit” in German). Spirit is of three kinds: the human spirit which combined with body make up a human being; the defined spirit that has no body, such as angels; and the infinite Spirit, or God, of Whom the Third Person is the Holy Ghost. When someone asks whether ghosts exist, he usually has in mind the first kind, a human spirit. Hence Father John Hardon defines a ghost as “a disembodied spirit. Christianity believes that God may, and sometimes does, permit a departed soul to appear in some visible form to people on earth. Allowing for legend and illusion, there is enough authentic evidence, for example in the lives of the saints, to indicate that such apparitions occur. Their purpose may be to teach or warn, or request some favour of the living” - Fr John A Hardon SJ, Modern Catholic Dictionary (Garden City, New York: John A. Hardon, © 1980) published by Doubleday and Company, p. 229.
The last sentence of Father Hardon’s definition implicitly gives the Church’s teaching on ghosts. Appearances of ghosts are understood with regard to our salvation. Ghosts can come to us for good, but we must not attempt to conjure or control spirits. The Church teaches that spiritism, ie seeking recourse or power from ghosts, is contrary to the virtue of religion (the Commandment: “You shall have no other gods before Me”):
“All practices of magic or sorcery, by which one attempts to tame occult powers, so as to place them at one's service and have a supernatural power over others — even if this were for the sake of restoring their health — are gravely contrary to the virtue of religion. These practices are even more to be condemned when accompanied by the intention of harming someone, or when they have recourse to the intervention of demons. Wearing charms is also reprehensible. Spiritism often implies divination or magical practices; the Church for her part warns the faithful against it. Recourse to so-called traditional cures does not justify either the invocation of evil powers or the exploitation of another’s credulity.” (Catechism of the Catholic Church, number 2117).
Thus, while the Church recognises the existence of ghosts, Catholics are not to intentionally seek them out — for good or for ill. Another theory about ghosts is that these spectres are actually a space-time-continuum replay of events that have happened in the past. This theory is possible, and is the most likely one that could fit into the belief system of the majority of Christians. Finally, there is always the strong risk that a "ghost" might, in fact, be a demon masquerading as a departed soul to torment and deceive the living. The Catholic Church unequivocally teaches that angels and demons are real personal beings, not just symbolic devices.