I've thought about this, because I'll sometimes get very loud (or inaudible at other times) without realizing it. I have been called on this multiple times. I think that NTs tend to instinctively modulate the tone and volume of their voice to fit the situation at hand, while individuals on the spectrum tend to lack that instinct.
I think that I tend to get loud when especially anxious and/ or stressed. Conversely, I also may get loud when I'm talking about something I'm really excited/ passionate about. I'm likely to become inaudible when I feel like I'm "supposed" to speak, but *really* don't want to, or if I'm feeling especially down on myself.
I find that I need to consciously think about the volume of my voice if I want to "get it right." Often, there are too many other things on my mind, and it just doesn't occur to me to do that. Who has the energy/ presence of mind to constantly monitor the volume of his/ her voice? I really haven't figured out how to resolve this particular issue.
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"And I find it kind of funny, I find it kind of sad./ The dreams in which I'm dying are the best I've ever had."