For quite a while I have been struggling to find the right voice to tell someone else's story. As a writer I have my own voice, my own vocabulary, my own style. But when you write a biography, you want to give the reader a flavor of who the person was. One way to do that is to find a voice that reflects the character and and can convey the conflicts that occured. Of course, you are not going to write in Early American dialect if you are writing about Paul Revere, but your prose might be clipped and quick like his race across the countryside.
Jonah Winter, for example, captured artist Frida Kalo's bold fierceness with short bursts of text in Frida.
Carole Boston Weatherford uses a first person narrative to tell the story of polar explorer in I, Matthew Henson. It seems to be the perfect vehicle to show the contrast of the world he lived in and the world he dreamed of -- "I did not take a job as a stock boy at a men's story to work my way up the ladder to clerk. I yearned for wind at my back."
Using voice as a vehicle to tell the story allows you to be more lyrical than straight forward prose, however, it might constrain you too. It is harder to weave in facts that a student, teacher, librarian or reviewer might be looking for. You might need to create an author's note or give more info in the back matter.
It is a delicate balancing act, and I am still falling off the rope as I play around with voice.