1) When you think, do you hear your own voice in your head?
Depends on how you define "thinking" doesn't it? To think in words, yes you need to hear the voice in your head. To think in images and emotions, no you don't. In fact I've found the latter form can occur at a much faster rapid than the former. Like reading aloud slows down your reading speed, thinking out your words slows down your thought processes?
2) Why is it there?
Cause the monkey's placed it there.
3) Is it your thoughts or is it just a translation of the thoughts?
Both when I need to later articulate it... like typing up this post, I translate to that voice inside my head. You first get a mental glimpse of an indescribable "idea" then the voice takes over. It dictates what I which words I ought to use. However once the words get started... the words are a part of thought.
4) Have you actively tried to think in some other form? (example: think in images)
I thought thinking in images comes naturally like that voice inside your head?
5) Do you think the voice limits your thinking efficiency?
See 1). Yes I do. It's slower and it limits you to thoughts that can be described by words. What does snow tastes and feels like? You can picture it easily in your head but it's impossible to describe it fully with words.
I believe there has been threads about this... but darn if I can find them.