Is your voice like handwriting? Can you improve the quality and pleasantness of your default voice (pitch, intonation, etc) like you can with penmanship so that the changes you focus on become the default? Or will you always be in some way "singing" your words?

You can take vocal/speach lessons to change the qualities of your voice. Here's the http://www.self-expression.com/coaching_speaking_voice.shtml one such person. My parents both grew up in Pittsburgh, PA. There is a http://en.wikipedia.org/wiki/Pittsburgh_English, frankly it's pretty blue collar sounding. My Dad's is still pretty much there (despite being a totally non-blue collar guy), Roooosevelt, Arn (for iron), Still (for steel), etc. My mother, who grew up no more than 2 blocks away from him, took vocal lessons to get rid of the accent and she has no trace of it at all. Vocal coaches in Hollywood work with people all the time on voices for characters, or in helping actors achieve a more pleasant speaking voice. I think we do this ourselves unconsciously. I speak differently to my husband, than I do to a girlfriend.

to a certain extent, that's what voice coaches do. One of the most famoushttp://www.dailymail.co.uk/news/article-2055214/How-Laurence-Olivier-gave-Margaret-Thatcher-voice-went-history.html I know of is http://www.youtube.com/watch?v=28_0gXLKLbk, (not the best example I could find) partly as the result of the fact that in the debating that passes for political interaction in the House of Commons she was perceived to be shrill and hectoring. a quote from the first link: 'Soon the hectoring tones of the housewife gave way to softer notes and a smoothness that seldom cracked except under extreme provocation on the floor of the House of Commons" You might read "The Gender Of Sound" an essay from Anne Carson's "Glass, Irony, God" book It is most definitely possible but I'm not sure of the extent to which it becomes unconcious like handwriting. Even with handwriting we are concious of being a bit more careful with important messages so it may be similar in that people who train themselves to speak differently are more concious of their output when it is important.

Sure. Talk to a speech therapist. Despite being born and raised in the Deep South (Mississippi) and spending my teenage years in New Orleans, I have a neutral accent rather than the southern drawl or the Yat accent. This is because I had a lisp when I was a kid and my mom sent me to a speech therapist for that and, as a byproduct, I usually have a neutral accent. (It's not a conscious effort on my part). But it is hilarious when I get super drunk and Cletus the Slack-Jawed Yokel comes out, because the drunker I get, the more the drawl comes out. Interestingly, and I don't know if it's related or not, I tend to pick up the rhythm of wherever I live or am at the moment. When I lived in Oslo, I picked up the sort of bouncy Norwegian English patterns. Even when I do a stopover in London for a couple hours, I'm told I sound vaguely English by the end of it.

I am a speech therapist and used to work with voice clients. All of voice therapy is retraining your voice habits to speak differently. You definitely can retrain yourself so that speaking in the new way is automatic but it takes a lot of hard work - my clients were asked to practice for 10 minutes 3 times a day and that typically went on for at least 6-8 weeks. Depends exactly what you need to change.

I took speaking lessons. I have a naturally somewhat deep voice (for a woman) and I didn't like how it sounded when it was recorded, so, since I had to be on the phone a great deal, I didn't want people to avoid my calls or cringe at my voicemails. Changed my life, or at least part of it, and I'm always glad I did. Now it's deep, but not nasal, and I speak more slowly.

What would be the name of the person you would see or hire to help you with this? Speech therapist? Speaking lessons? "Vocal Training" seems to be related to singing, though I'd imagine there is probably overlap.

You can change your voice to a degree. But your modal voice, that is your 'natural speaking voice' is somewhat fixed, and for most humans rests around about a fourth or fifth higher than their lowest comfortable singing pitch. And this your optimal speaking register for a reason - the size of your vocal folds, the sort of default position of your larynx largely determines this. That isn't to say that it can't be changed, but changing your natural pitch register might not be the greatest for your throat. You could end putting stress on your vocal folds overtime, and causing damage in a variety of ways. Changing your accent, dialect and the like is much easier, because that depends much more on your articulators, and you can change your articulation very easily with practice and with little downsides. Changing how you move your teeth, tongue, lips, velum is by and large much simpler than changing how you move the structures of your larynx.

I took a college course called "Voice and Articulation" and much of what we did was just that--practicing changing all of this stuff. But like Lutoslawski said, speaking regularly outside of your natural pitch range is bad for you.

Practice, practice, practice. My natural range is alto/tenor. When I was a choir director and sang frequently to show the sopranos what I wanted, I could comfortably sing a high G. In the old days of radio, there was an unmistakeable "NBC voice". Deep and authoritative. That is achieved by learning to speak from the diaphragm. If you are serious, get a voice coach.