I've always talked too loud and too much. As a child, I over-enunciated... my voice had an almost spitting sound to it.
If I'm talking about something I like, or if I'm in a particularly happy mood, my voice gets very sing-songy and cheerful.
If I'm in an uncomfortable situation or if I'm being especially respectful or polite, my voice becomes higher and softer. Not necessarily quieter, but... "demure" is a word that comes to mind. Most people mistake me for a child if they hear that voice over the phone.
If I'm in a conversation I don't want to be in, I mumble, and my voice does become a bit monotonous. I'm incapable of faking an "engaged" tone of voice, even though I can fake the body language and the correct words.
If I'm actually in a foul mood or angry, my voice drops and takes on a more masculine, menacing tone. I sound COMPLETELY different when I'm angry than I do when I'm happy or sad, which tends to freak people out and make them think MPD. >.>;
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"Why am I sticky and naked? Did I miss something fun?" - Philip J. Fry
The difference between madness and genius is that a madman looks into the abyss and averts his gaze; a genius looks into the abyss and describes what he sees.