The eternal question. Does it matter? Yes, it apparently does, especially when you hear editors and agents at conferences say, "A story with a great voice and a weak plot can be fixed; a story with a great plot and a voice that isn't working can't be fixed."
So, are they right?
Yes and no. Some stories emerge from voice. John Marsden says that he began the War series (which starts with Tomorrow When the War Began) when he heard Ellie's voice in his head - Ellie being the narrator. It's often the way it works for me. I started a story about a girl called Tracey Binns and her voice just took over - she was very demanding! But I have written other stories where the voice is not nearly so strong. With chapter books, it's not such an issue. In fact, I have a chapter book where the voice is probably stopping it being published, because the kid is pretty nerdy and weird.
In class, I try to get the students to do a lot of work on their characters, not because I think they need to know every single thing about their character's life (although it helps) but because in writing and imagining their main character, they can often "fall into" the voice. We do free writing, imagining our characters telling stories that begin, "Let me tell you how it happened ..." or in YA, perhaps, "This is how it went down ..." One good exercise I recommend is to interview your character via free writing, sit them down at a table, ask them questions and then let them answer. All kinds of strange and wonderful things can come out of this, including things that you didn't know were part of your character's life.
Back to the question - can you fix a weak/uninvolving voice? Yes, I think so, but it requires several things to happen:
1. That you put aside the manuscript and forget entirely about it.
2. That you focus on your character and spend a lot of time writing about them and writing things in their voice - role playing, imagining their world, and then working your way into seeing the world through their eyes. How your character sees/understands/filters/judges the world around them is, to me, an intrinsic part of voice.
3. That you create a whole, real life for your character - their family, friends, school/work, lovers, enemies etc.
4. And then you focus on their dreams, goals, ambitions and fears. Get your character to write secret thoughts about these things. Note I said "get your character to write" - by now, if you have really delved into who your character is, it really will be her or him writing about what they fear most, or what they want most in their lives.
If you haven't captured voice after doing all of those things, you need to ask yourself why not. And weird though it may seem, it might just be because you are too afraid to let your character be "real" to you.
Enough psychoanalysis-type stuff for one night.
I'm off to read some Peter Temple.