I rarely talk to anyone anymore unless I'm spoken to first - and when I do talk, I'm usually quiet and monotone, as well. For a similar reason (discretion - I grew up with a bunch of gossipers and I prefer privacy when I'm talking to people... it's to avoid other people trying to butt into the conversation and talk over me, too).
It's weird because when I go hang out with one particular friend of mine, I'm the total opposite. Still quiet, but much more hyperactive and talkative / random.