First, you need to be able to differentiate between your imagination and true clairaudience. With true clairaudience, most agree that you hear the thoughts/voices as clear as if it were a voice whispering or speaking into your head.
What trips most people up is that they will hear several sounds together, such as the hum of their computer and then a pop in a wall. This leads their imagination to try and make sense of it and give them a conjoined sound that's mumbled. They think this is something trying to speak to them, so they then start "making" something happen in their mind.
Now, as far as "inner voices", there are your chakras and "higher self". You may or may not get to the point of actually hearing them. You can concentrate on a chakra, such as your 4th (heart) chakra as a glowing orb or triangle radiating out bright green light and ask it a question. It'll respond in feelings most of the time.
With your "higher self" it could be a thought/conscience type of message that just appears as if you were the one thinking it. I, myself, have felt it sort of as a thought behind a thought. It's only happened a couple times where I " knew " my higher self would respond. This is generally when I had meditated on requesting an audience to discuss a specific issue or goal. I'd repeat the request over and over with pauses filled in to see if a thought or voice would enter my mind without me implanting my own thought. Sometimes it'd happen then, sometimes it'd be me waking up in the middle of the night with the reply in my mind. At the later example you can request an answer to another question and usually "feel" the answer right behind the question you asked.
Regardless, you have to be able to discern truth from fiction, and also truth from you " wanting " or " making " something happen. Most people " really want " a certain reply subconsciously, so it's what they get back.
You might want to read over this URL: http://kundalini-teacher.com/symptoms/voices.php
I have found that it really explains the basics of voices pretty well.