This is a good point. There are often differences in how people of different genders (and nationalities, interest groups, classes, educational levels, etc) express themselves regardless of the format, and modulating their voice doesn't really hide that.

For example, I've run quite a few forums before, and I'm pretty confident I (and quite a lot of other forum admins/moderators) could reliably judge someone's gender through their manner of writing and the (non gender related) info in their profiles.

So if you wanted to really remove possible gender 'bias' from the occasion, you'd have to go much, much further. You'd have to give people a way to communicate which reduces their responses to preset answers, that's entirely conducted through text on a screen and make sure all personal information is filtered out as well.

There's always going to be a trade off here. On the one hand, I'm sure the system used in the Luigi's Mansion Dark Moon Scarescraper would completely eliminate any potential gender bias, but that's only because it limits chat to 'over here', 'good job', 'thank you very much', 'help' and 'hey'. On the other hand, an interview in person would give you a lot of information about how well they could do the job (and let you test it), but then you've got psychological biases to deal with.

It's a tough one really.