The Rookie's voice Has not been heard in any of the demo's, any of the briefings, or mentioned in the game informers, But it is confirmed that Nathan Fillion will voice the Rookie. Many of us do not know what he sounds like. the above link is a video from firefly. which he plays Captain Malcom Reynolds. Enjoy!
NOTE: Chances are that he isn't going to talk that much throughout the game, with him being 'The Strong Silent Type' and all...
EDIT: Actually its Buck's voice. there was confusion. my bad.
[Edited on 06.26.2009 12:58 PM PDT]