One thing I'd suggest, as it helps me, is listening to specific songs that you associate with your character and her outlook on life.
I do this too, it's a great tool.
"Voice" is (IMO) one of the most elusive things to pin down, but in the end I think a lot of it comes down to how well you know your characters and their world. I started my current MS with a clear idea of my narrator's voice, and it didn't change *significantly* throughout the story. BUT, I'm also a pantser which meant that I learned more about her as I wrote... and in edit I did have to go back and do some tweaking to the early chapters to bring the voice into line with how it had evolved. For instance, my narrator definitely has a sarcastic streak, but it toned down a bit as the story went on and I felt I needed to trim a couple of spots early on where it was more intense than it needed to be.