Why is it that I have different voices for different activities?
Lots of people have a "telephone voice" - especially when they don't know who is on the other end of the line. This morning I taught a group of undergraduate students... using my "lecture voice"... and I know someone whose voice acquires a Western twang when he's feeling awkward in social situations.
My theory is that these alternate voices provide us with alternate personae when we're not 100% comfortable - it's a way both to hide and protect our true selves from injury. Although we usually interpret these feelings as shyness, it's really a form of fear - when you get right down to it. Although it's irrational, we subconsiously give the people who cause these feelings a false target to aim at. Just in case.