While they are typically thought of as being seen, there are times in which apparitions have reportedly been heard, as well: Reports of phantom voices, footsteps, and other sounds mimicking human activity do tend to surface on an occasional basis, mostly in haunt cases but also in other kinds of spontaneous spectral encounters. And indeed, surveys tend to indicate that auditory experiences are the next most frequently reported form of spontaneous apparitional encounter, behind visual sightings.
For instance, in surveying a collection of around 850 reports, British psychical researchers Celia Green and Charles McCreery found that around 37% of the encounters were auditory in nature.  In a more recent survey of 449 reports, Icelandic researcher Erlendur Haraldsson found that 28% of them involved phantom sounds. 
One particularly intriguing thing about some of these auditory experiences is that they are said to manifest in form of human voices which seem to convey subtle information or a warning about things to come in the near future. One example comes from the extensive collection of reports of spontaneous psychic experiences gathered by the late parapsychologist Louisa Rhine:
A number of tourists [on] Labor Day...were waiting their turns at Niagara Falls to board the Maid of the Mist. One girl was about to step on the plank when she heard the voice of her dead sister saying to her, "Don't go, don't go." It was as clear as if her sister had been standing next to her, but she had not been thinking of her sister for days. Along with the voice, the girl felt a pain in her right arm and leg, and as if paralyzed, she watched the ship pull out.
A short while later, the Maid of the Mist returned, bringing in some injured passengers. It was the first time an accident had occurred to the ship. High wind and waves had separated the mast, which fell and cut the faces of some of the passengers. [3, p. 142]
Dr. Sally Rhine Feather has continued the work with spontaneous reports that her mother Louisa Rhine had started, and she came across a letter from a female correspondent that contained a personal account of experiencing a similar kind of example of a spectral voice, this time involving a negative event which had a more positive outcome:
I am an only daughter, and when my father was alive, I could count on him whenever when I was in need of help. After my father died last June, I felt really alone and somewhat afraid. My husband had been ill for two years, and though he is out of the hospital now, we are badly in debt. Our car payments were behind three months, and two weeks ago they came to repossess the car. While I was talking to the man at my front door, I heard my deceased father say, just as clear as my own voice, "Tell him to wait until Friday." Without hesitation I said to him, "Wait until Friday, and I'll pay you." He agreed and left. After he had gone, I wondered how I was ever going to raise $280.
Thursday afternoon, my dad's lawyer called me and told me to come to his office and pick up a check for $417. The courts had collected from an old account owed my dead father, which I was to receive. When I hung up the phone, I looked up and said silently, "Thanks again, Dad." [4, pp. 263 - 264]
It seems that in these two reports, the phantom voices heard by the witnesses were offering warnings or information which later turned out to be accurate, and which the witness could not have likely known about ahead of time through simple logical inference. Could the spirits of their deceased relatives really have been psychically speaking to them from beyond the grave?
On the surface, that would appear to be one possibility to consider, although there is another which might be seen as being just as equally applicable to this situation:
Quite a number of people have said they've had psychic experiences in which seemingly became aware of an future event - a phenomenon known as precognition. Most often these experiences tend to manifest in the form of dreams, although there are also occasions in which they've been said to be akin to an intuitive "hunch" or a kind of "sudden impression" which might occasionally manifest in a sensory form (such as a voice - see e.g., Ref. 5, pp. 251 - 252). In light of the considerable amount of experimental evidence that has been gathered for this phenomenon over the past eight decades , perhaps some of the phantom voices which seem to convey accurate information about the future might represent an auditory-based precognitive impression.
Which of these two possibilities might be more plausible? It can be difficult to tell with certainty at this point (from a purely objective stance), and this leaves the question of interpretation open to individual consideration for now. Perhaps continued study of this intriguing phenomenon will "speak" toward this issue further. :-)
 Green, C., & McCreery, C. (1975). Apparitions. London: Hamish Hamilton, Ltd.
 Haraldsson, E. (2012). The Departed Among the Living: An Investigative Study of Afterlife Encounters. Guildford, UK: White Crow Books.
 Rhine, L. E. (1981). The Invisible Picture: A Study of Psychic Experiences. Jefferson, NC: McFarland & Company, Inc.
 Feather, S. R., & Schmicker, M. (2005). The Gift: ESP, the Extraordinary Experiences of Ordinary People. New York: St. Martin's Press.
 Rhine, L. E. (1956). Hallucinatory psi experiences: I. An introductory survey. Journal of Parapsychology, 20, 233 - 256.
 Mossbridge, J. A., & Radin, D. (2018). Precognition as a form of prospection: A review of the evidence. Psychology of Consciousness: Theory, Research, and Practice, 5, 78 - 93.