Oh god, all the time, and I'm doing it more and more as my life goes on -- but that may be because I'm permitting myself to do so, since it dawned on me that it helps me in many ways.
I don't do it in public around people -- I know enough to control it because other people think you're a raving lunatic if they see you talking to yourself.
I also do not have a "conversation" with myself, like question and answer style -- that's not what I do. I simply speak out my thoughts almost as if there is another person to listen to me, though I'm very clear that it's only myself listening.
I know precisely why I talk out loud to myself: because it helps me order my thoughts and make more sense of what I'm thinking and feeling, than if I'm just thinking inside my head like most people. It functions the same for me as writing one's thoughts down on paper -- it's a clarifying technique.
I also do it to "rehearse" things I'm going to want to say in an upcoming conversation I know I'm about to have with a specific person. It helps to script-out ahead of time some of the responses I need to give or things I need to say, as I find that I can suffer from slow processing when "on the spot" in a conversation that may be stressful to me.
I also talk out loud after the fact, when a conversation did not go well and I'm trying to make sense of what I could have said better. Because of my processing issues I often do not manage to stand up for myself, advocate well, say something in such a way that could have made things go in a better direction.
So I'm always doing a post mortem on those events, and I pace around my apartment trying out loud the other things I could have said, or something I want to say better the next time.
Another aspect is that when I feel very stressed I tend to talk out loud more from sheer agitation. I always know when I'm not as happy or calm as I even think I am because the talking out loud of a non-preparatory kind gets worse.
In none of these scenarios do I hear voices or think I'm talking TO someone though. I am clearly only "thinking out loud" and I know it.
I don't think it's "not healthy" because I believe that it IS a legitimate coping method for some types of stressful thoughts, and a way to crystallize one's thoughts if confused or trying to figure something out, similar to, as I've said, writing it down. I do journal too, but I find that talking out loud also helps.