What is 'Voice'?

This is a question that came up actually quite unexpectedly.  I've been writing a daily writing prompt, at least 500 words of a story or idea for the past couple of days.  Four prompts, total.  Pretty cool ones, if I do say so myself - there's the very beginnings of a story about the University of War, the end of the world, a vivid dream, and an overly clichéd character - and as I've been writing I've had to ask myself why all of my writing seems to sound the same.

These story snippets are all different genres, written from the points of view of four entirely different characters, with different things going on in their lives.  But for some reason, there's a common thread - they all sound like me.

This has been an issue with me in the past, particularly with my first novel, and is something I'm trying to correct as I edit my second.  I guess I just didn't realize, until I started doing these, how much everything kind of sounds the same.  Not just the dialogue, but the descriptions, the words.  Is this my voice?

There are two ways I'm looking at this.  One, it's bound to happen.  I write, well, often, and there is no way to not develop a voice when you write.  It's your style, your signature.  It also means that I'm becoming more aware of how I write, and I'm taking a step back to examine my work, which is definitely a positive for me.  On the other hand, I don't know how thrilled I am with this voice.  I feel like, if I'm taking the same approach to writing every time I sit down to write, than I'm not learning anything, and I should be.  I am not an expert at this.  I consider myself good.  Maybe well above average.  But I don't think anyone is good enough to get complacent.

So here's the plan.  I'm going to take a few steps out of my comfort zone.  Maybe write some things that don't come as easily to me, write in a style or in a medium unfamiliar.  Poetry has been suggested.  So has playwriting, or writing a page of a comic.  Things that I don't know much about, that it would be a good idea to learn.  Maybe writing a story from a completely foreign mind, or just keeping the thought in my head, as I write, to do it differently than usual.

I'll give that a shot, and if any of it turns out halfway decent I'll post it on my off days.  I need something to keep the internet occupied.