My inner voice is often the neutral ethereal voice mentioned by other posters, but I am able to conjure up the more female-sounding version from time to time as well. I would say once your outer voice becomes consistent and regularly used, that's when your brain starts to capture the sound and essence of your voice and can apply it to your inner voice.
As an exercise - try to imagine a celebrity, friend or family member's voice reading something, such as a post on the forum, and try to hear the voice inside of your head. After hearing enough input of their voice, you should be able to simulate their speaking voice in your head, much in the same way you can read any sentence in your native language even if you've never seen it before. I speculate that this is what our brains do to find that inner voice, and since the largest source of voice input is our own, that influences the sound of our inner voice.
