Hearing voices in the mind is the most common type of hallucination in people with mental health conditions such as schizophrenia. The voices can be critical, complimentary or neutral, and may make potentially harmful commands or engage the person in conversation. They may give a running commentary on the person's actions.
The experience is usually very distressing, but it's not always negative. Some people who hear voices are able to live with them and get used to them, or may consider them a part of their life.
It's not uncommon for recently bereaved people to hear voices, and this may sometimes be the voice of their loved one.