From what I've gathered through my 2 years of voice training, the main reason I got this far with my voice is because I forced myself to look at this from a technical anatomical perspective, rather than thinking all I had to was get used to speaking feminine and eventually I'd wake up one day with a beautiful female voice. I don't see how that will ever happen for anyone who is trying to convincingly alter their voices from one gender to the other. You have to exercise those muscles(to get rid of fat) so that you can use the muscles and exercise them more to get your ideal voice.
We'll compare it to losing weight. You are not going to get that ideally beautiful body with toned abs, ass ,arms and face just because you're on a diet and exercise everyday, BUT you know that its possible and you don't want surgery because you're scared of the after-effects or because it cost too much. What you have to do, is look at what the surgery does and try to mimic those result through hard work and exercise, no matter how extreme it will be. The same thing applies to your voice. You can physically change which muscles you use so that your voice can sound completely male or completely female.
There are muscles in your neck, chin, and facial areas that help you produce speech. Men and women use these muscles differently and that is main difference between a male and female sounding voice. There are things you can do with your voice in male mode that female voices will not be able to imitate because their muscles are not in a position to do those things & vise versa.
We know that puberty causes the physical voice changes because it gives boys a release of testosterone that gives the muscles enough strength to expand and allow the adams apple to drop. This results in the male voice getting deeper. Since the drop of the adams apple is the cause of this new deep voice, what we need to find out is why the adam apples(larynx-voice box) dropped in the first place.
Before puberty, Boys and girls voice boxes are upright and basically in the same position. Naturally, both sexes are using their muscles the same way, The reason why boys adams apple drop is because they have more muscle strength ,from testosterone , given to them through puberty and it makes it easier for their muscles to give way for the drop. The same thing happens to girls during puberty. They also receive an increase in testosterone but it is not significant enough to give them enough strength for their voice box to drop like that of a males.
But because of the dramatic drop of the larynx, males will feel a physical discomfort in their throat muscles and voice during puberty. This discomfort happens because, not only is the larynx growing, but its forcing the muscles to expand. This is why a boys voice will go up and down during puberty. The neck muscles are trying to find comfort around the new larynx position and the brain is trying to produce a voice that sounds 'good' to your ears.
The way society expects us to talk plays a very important role in the muscles we choose to speak with. Talking in a deeper voice forces the larynx to a lower position, where as speaking in a higher voice keeps the larynx in a higher position. Boys are expected to sound deep, girls are expected to sound higher than boys. These are psychological changes that children go through either knowingly or unconsciously. This helps familiarize the brain to tell which muscles in the face, neck, and chin area should be use for natural speech.
--After puberty, everything that you did to your voice is enforced. The muscles that you abandoned during puberty are not used as often and are not exercised enough during natural speech to remain in good condition. As the years progress, fat builds up around those muscles and makes it that much harder for those muscles to assist in lifting your larynx high enough to produce those clean pitchy sounds. Its hard to exercise the muscles in your neck, face, and chin areas so it will be hard to lose the fat around the muscles that are key involvements in female speech.
It is possible to train your muscles to sit the larynx BACK into a female position. This helps cut down on male resonance(chest voice) but there are many other muscles that have to exercised before this is possible.
I am not a doctor,my knowledge of this comes from my own research and experiences with changing my voice from male to female. I didn't realize this until June of 2008, but after I started exercising to 'restore my voice' back to a pre-puberty neutral state, it did wonders.
I am still exercising certain muscles in my chin area that I believe are preventing me from speaking certain sounds and the results I'm getting are very assuring.
I hope the understanding of this posting helps someone out there as much as it has helped me when I realized it.
It takes time. It takes patience. It takes intense neck, face, & chin exercises, and a whole lot of drinking water. ;)