I think voice is more subtle than that. I could be given a scene outline and character sheet and write that scene in four different ways myself. That wouldn't mean I have four different authorial voices; it would just mean there are an infinite number of ways to write a scene, even with the same character and outline.
Sure, but could you write the scene in an infinite variety? Some collected subset of the set of all possible ways the scene could be written is all you are capable of. That set is your voice.
If your worried about a message you want to convey, you can give it through the MC learning it from experience. If it's just annoying because you want to throw out some really great quips you came up with, have a character say them.
Then I think my voice is pretty varied... @Tenderiser says she can spot my writing, and I can see her point. But I think that's the way I'm currently comfortable writing... I don't think it's the only way I ever could write, or even ever will write. It's just the way I currently do write. Is there a temporal element to the definition of voice?
Maybe that's a good thing? I don't think mine is that varied, but I haven't been writing very long. I'm still focused on just trying to write anything that sounds halfway decent. Think about one trick actors like Robert Downy Jr. who seem to play the same guy in all their movies (at least that's my impression of him), while guys like Brad Pitt and Johnny Depp can put on wildly different performances.
These subjects lend themselves well to philosophical discussion. So much can change based on the slightest change of interpretation. It's fascinating. And this thread has really helped me think through all of this. Big thanks to you all.