There is a good discussion of "voice" in the liner notes of a new album that Renee Fleming just released. She is probably America's best soprano at the moment, and she has switched genres to do covers of rock songs written by the likes of The Mars Volta and Leonard Cohen. It is an excellent effort.
She discusses how singing in this new genre required her to re-learn everything about singing this kind of music. She is two registers lower than her operatic voice, and she had to learn hos NOT to project but instead use the microphone to add nuance.

Gave me a whole new perspective about "voice."