How is communication with the dead even possible?
The idea that you die when your brain dies is just a theory (called materialism). An alternative theory is that consciousness is separate from the brain, survives death, and can communicate with the bereaved or a medium.
A relationship clearly exists between the physical brain and the mind/self/consciousness (what makes you you). When the brain is injured or damaged, the mind functions differently. However, this does not prove that the cells and chemicals of the brain make mind. Correlation does not equal causation. Alternatively, mind may be like a signal and the brain like an antenna. Without the antenna, the signal can still exist. Evidence from several fields of research support the reality of this signal/antenna explanation (see also: Fact Sheet: Support for the Survival of Consciousness Explanation).
Is hearing from the dead normal?
Though hearing voices with no physical source is a notable symptom of serious mental illness, researchers at Yale have reported that the phenomenon of hearing voices exists on a continuum from health to disease (Powers, Kelley, & Corlett, 2016).
Furthermore, modern bereavement researchers view spontaneous experiences of contact with specific deceased loved ones as a natural part of the grieving process. These experiences occur in all types of people regardless of age, gender, ethnicity, education level, income, type of death, time passed since the death, or socioeconomic or religious standing (reviewed in Beischel, Mosher, & Boccuzzi, 2014-2015; see also: Fact Sheet: The Four Types of After-Death Communication Experiences).
Experiences causing distress should be brought to the attention of your healthcare provider immediately.
Doesn’t this change everything?
Scientific research on topics like life after death and communication with the deceased faces an on-going struggle against a scientific paradigm bent on denying people’s extraordinary experiences and challenges current ideas about the mind’s relationship to the brain. Laboratory studies demonstrate that certain mediums can report accurate and specific information about the deceased with no feedback during or after the readings and without using fraud or deception (see Fact Sheet: Testing Mediums’ Accuracy Under Controlled Laboratory Conditions and 'What does the scientific community know about mediums?'). Thus, mediumship and related abilities are real (see also: Why is mediumship research important?), but this does not go against what is currently known about perception, psychology, basic physiology, biology, geology, astronomy, sociology, fundamental physics, quantum physics, or relativity (e.g., Braude, 2019).
Beischel, J., Mosher, C. & Boccuzzi, M. (2014-2015). The possible effects on bereavement of assisted after-death communication during readings with psychic mediums: A continuing bonds perspective. Omega: Journal of Death and Dying, 70(2), 169-194. doi: 10.2190/OM.70.2.b https://windbridge.org/papers/BeischelMosherBoccuzzi_AssistedADCs.pdf
Braude, S. E. (2019). Science doesn’t dictate what’s “impossible.” Journal of Scientific Exploration, Vol. 33(4), pp. 541–548. doi: 10.31275/2019/1693
Powers III, A. R., Kelley, M. S., & Corlett, P. R. (2016). Varieties of voice-hearing: psychics and the psychosis continuum. Schizophrenia bulletin, 43(1), 84-98.