In the Asylum of the Daleks (Dr. Who - Series 7), Oswin guides the Doctor and Rory through the Asylum planet maze through the voice speakers (megaphone). It is eventually revealed in the end that she is a Dalek herself, and what the viewer saw throughout the episode was her "dream state".

Why doesn't her voice sound like a Dalek when she was communicating with the Doctor or Rory over the speakers?