I think I know what you're talking about. I'm usually not a shy person at all and I'm used to talking in front of many people, etc. In therapy I've noticed how sometimes my voice becomes very, very quiet. So quiet in fact that my T will have to ask me to repeat myself.
I'm not sure I completely understand it (and will think more about it) but I do think it might have something to do with shame...

(Also, I sometimes have trouble speaking at all. Perhaps this is linked to my voice being quiet?)