There's a rather credible (in my opinion) theory that everyone "heard the voice of gods" a mere 3000 years ago. Your left and right brain hemispheres have their own thoughts, and it's because of the link made by the corpus callosum that they "feel like" they think together, but we know that they don't because of
split-brain patients who have had their corpus callosum cut... tests have shown that each half of the body, if allowed to express itself in writing, has different thoughts and experiences. But sometimes the parts of the brain, the ones responsible for recognizing these "two selves" in the brain and making them coherent, malfunction. In that case, one can often hear the separate thoughts and will think it's somebody else inside their head -- a demon, alien, god, separate personality, etc. And since the corpus collosum is on top of the rest of the brain matter and seems relatively new, along with the fact that atheism was practically unheard of 3000 years ago and people regularly wrote about literally hearing a god's voice, it would appear that everyone had this "defect" at one time.
This isn't to say that this is necessarily responsible for your own experiences hearing the voice of God, but it could be. One strong hint that it's your own thoughts rather than someone else's is whether or not it disagrees with you. Generally, people don't claim to have talked to God and had God tell them that they were wrong, otherwise we'd have answers to the pressing questions posed on this forum regularly (always saved or not, pre-trib or post-trib, KJV superiority?, etc.).