The player character Casey's voice sounds different in this game because
Steven Petrarca did not return due to a disagreement over pay. Reusing his audio clips from the previous game would have still required paying him at a rate too high for the reduced budget of this add-on game (which was released as a free internet download), so the player's radio commands were rerecorded with somebody else's voice. Since the game's credits only show a list of names for the voice cast without stating which roles they played, it is a mystery who voices Casey in this game.
Heather Stephens (Stiletto) did not return either, but presumably didn't demand more money than the budget allowed so her in-flight audio clips from the previous game are reused.
