Every person is capable of hallucination. We push our minds to see things that aren't there and we call it visualization. Some people close their eyes to do this, other keep them open. I have to leave my eyes open, I need color to see things. Our senses allow us to precieve the world around us and the two strongest are sight and smell (smell can actually trigger some very powerful memories). One that is a little harder for some people to fake are sounds. When I say this, I don't mean impersonations, I mean hearing a voice that isn't there--a hallucination of the auditory sense.
All people hear voices in the wind, things like that, but not all people can hear a disconnected voice of their own making. A persons voice is as unique as their fingerprint and when it comes to writing and finding voices for our characters, this can be especially difficult. Some writers are naturally gifted with what they affectionately call writers schizophrenia. The voice comes to them and the character follows. Some writers don't get that at all.
How can you find that voice then? Try to visualize the character first. Do your character development and try to write it out in a way that the character is filling it out about themselves rather than you filling it out about them. Sure, occasionally, your character will lie about themselves because we all know we don't know ourselves as well as we'd like to believe, but after they have filled it out, make notes about what is accurate for them. After you've visualized them, try turning them into an imaginary friend. Sounds silly. Many people had imaginary friends when they were little, and giving into that may be a bit difficult, but if you can accomplish it, it makes it so much easier to hear that voice.
One other voice is difficult to find when it comes to writing, as well. The voice of the narrator is a big problem for a lot of people. When you're writing a children's story, you want a voice that is going to speak to children. You don't want to talk to them the way an adult would, so what do you do in that situation? I like to treat the narrator as though they were any other character. Do a character development list for them, even giving them a body to go along with that voice. If I can make them as "real" as my other characters, I can hear their voice and they can tell me the story that they want me to write down.
These techniques may not work for everyone. I can only say what works for me. How do you work on your character's and narrator's voice?