Rosy, I know exactly what you mean (though I'm still not really here...) about this question, but I think it probably solves itself, in that you can't help sounding like yourself, even when you think you're sounding like your characters, just as an actor is as once a character and themselves, or as a forensic handwriting expert can recognise someone's writing even when they're trying to disguise it. There are things you don't even know you do: an underlying quality to how you string a sentence and a paragraph together, the pace and rhythm, habits of punctuation. They're probably more obvious to others than they are to you. (I think I look very different from my sisters and my mother because it's the differences that I notice: the rest of the world thinks we look quite comically similar...). I suspect that if you mixed up some extracts of different voices from your work, say, with some extracts from someone else's, someone familiar with your work could pick them out. I think once you've absorbed tons of good writing, and moved beyond it into your own synthesis, your voice becomes individual quite naturally.
But I think when the trade talks about 'voice' in this way they're often also talking about something even harder to pin down, and less to do with the arrangement of individual words and sentences. It's about what you're saying, and how you're saying it, what you chose to tell of a story, as well as how you chose to tell it. And, again, I think that comes about quite naturally, once you're trying to say your own thing.
I think these two kinds of voice are one of the most important reasons for a writer trying to find their writing, and why I bang on so tediously about it on WW (and elsewhere). Come to think if it, maybe teachers and editors should talk about finding your own writing, not your own voice.
An analogy could be with handwriting. When I was growing up my mother taught us italic handwriting, partly in defence against the standard curly commercial copperplate our American school taught. At my primary school they taught us Marion Richardson, and working to a different model was a matter of virtuous family rebellion. It was only in a little book called
Teach Yourself Handwriting - aimed at adults who disliked theirs for one reason or another - that I came across the idea of handwriting as something that should be free of a model: that you have a natural, personal way that you write, and that the goal - through exercises for relaxation and then control, and then a model alphabet specifically for this purpose - was to find that ease through which your individuality of slant, joins, heights, and so on could emerge.
Sorry, longwinded, but you take the point. And I'm not really here - surely you've realised?
Emma