Why not just use text? There are many speech characteristics of gender that will never be found and masked by modulation.

Simple placements of words, parts of speech referring to the self, and other things are all much better clues to gender then voice frequency and pitch. For example, when speaking to some people who I know who are either naturally very high or low pitch, or are trans-gendered, there is a lot more information that can be contextually extracted from the content of their speech.

Really, just do text-based interviews. It will allow people to revise what they are thinking and really allow them to mask their gender.