If it's some random person, my own voice. If it's someone I've seen around often, the voice I imagine they would have. If I've actually heard the person, then their voice. And sometimes, just sometimes, Morgan Freeman's or Shawn Connery's voices.
This may sound weird but I start off reading them in my normal thought voice, then as soon as I get about half way through I try to figure out a personality that I think they may have and I assign them a voice that fits.