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?
First point: Oswin was not completely transformed into a Dalek, she was converted. This means that there is a part of her that is still purely Oswin. That is the part that has deluded herself that she is in her ship because she cannot handle being a Dalek.
Second point: Oswin was a genius and actually did hack all those systems within the Dalek defenses. She was not using a conventional intercom, she had taken over the speaker system and transmitting as she desired.
Therefore, because she was not communicating via an intercom, she was constructing the sounds passed through the speaker directly, and therefore could choose how she sounded. It makes sense that she would project the voice that she is most comfortable with. She seems to have gone through great lengths to maintain her humanity, and her projection of humanity. It only makes sense she would project her actual voice and not the Dalek voice she rejected.
Remember when Clara went into a dalek in S9? What if oswin was in a casing similarly, just sleeping and living in her dream, but she was still using her own mouth to speak. This would explain why she sounded like herself in the speakers, because it wasn't going through the dalek casing, but it sounded like a dalek in the room because then it was just going through the casing. As to why she wasn't speaking the same way as in S9, maybe the casing was just malfunctioning? After all the daleks who put it together were all nuts.
