Some of my earliest memories are my mother as she read to us. I remember the way she brought each character to life with only the sound of her voice. I think that's where my siblings and I learned to create voices. Get all of us in a room and you quickly learn that we have a unique way of communicating; we love to imitate inflection and sounds of voices other than our own. We seem to have a knack for putting words in other peoples mouths by imitating not just what we think they would say but how they would say it. This often is confusing for people that don't "get us". (Sometimes mistaken for multiple personality disorder.) In any event, we have all become expert observers of behavior and communication.
When the Harry Potter series first came out, I immediately began reading this to my children. This was long before Hogwarts was a household word. I found her characters easily brought to life by my repertoire of voices. So distinct were my voices that when the first movie came out, my children complained that the voice of Dobby was all wrong. These characters changed and grew up with my children which made them incredibly compelling.
The huge changes and growth that occur during the middle school years are ideal for creating characters with conflict.
Do your early memories play a role in your writing? Does anybody else create voices for their characters?