In the Prestige, when the identity of Lord Caldlow is revealed to be Angier, he speaks with a British accent instead of the American accent he's used all film. Which is the real voice?
The real voice of the character is the British one. Lord Caldlow is a British Aristocrat who changed his name and accent to hide his true origin, in order to pursue his dream of being a stage magician. This choice of this career, effectively in showbusiness, would have severely embarrassed his family. So he pretends to be the American Robert Angier.
From the script:
CUTTER: Dear God.
ANGIER: Hello, Cutter.
CUTTER: You're alive. (Angier nods) You're Lord Caldlow. (Angier nods again) How?
ANGIER: (shrugs) I learned an American accent from...
CUTTER: (Angry) How are you alive, Robert?! I saw you on the slab, for God's sake
Angier fixes Cutter with a steady gaze.
ANGIER: I magician never reveals his secrets.
(Note: I've not confirmed this is the exact wording used in the movie, sometimes scripts found on the internet are not the final versions used.)
One of the major themes of the movie, illustrated by the Chinese magician near the start of the story is 'how far would you go to be the best'. Borden hides a huge secret to the point of ruining his relationships and losing fingers. Angier / Caldlow has lived a secret life as well as using 'the machine'.