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The young american actress Chloë Grace Moretz already did the voiceover for the character of the movie. But Disney replaced it on the last minute in favor of Miley Cyrus because they thought that Cyrus' voice was more well-known for the american audiences and it would suited better the film cast for commercial reasons. Moretz only scenes in the final film where the ones where Penny was a young girl.

What do you think? I think that Chloë's voice would have suited better the role of Penny because at the time she was likely the same age as the character. But Miley Cyrus by those days was Disney's new rising star and she was way more famous.

Reference: http://disney.wikia.com/wiki/Penny_(Bolt)

I don't think either actresses were fit for the role. Grace, unless she's actually older, would have been too young to voice a older girl. Miley honestly sounded a bit too old for her character. I think Disney should of hired a girl was who around 12-14 years.