This started on Tuesday for me as well. It's funny I was alone on this until I saw this post. I just thought I kept hitting the speakerphone on accident but by the end of the phone call I'd forgotten it keeps happening.
I do have GoSMS, although it sounds like at least one person (fire n ice) has the same issue but doesn't have GoSMS. But still, they did release an update acknowledging the issue so hopefully its fixed now.
Still though, very annoying. I was in a class trying to quietly listen to my voicemail when it popped on and said "please enter your password" Everyone turned to me and I was like "yeah, I'm that guy, sorry"