I have my client set to speak with the male voice. I find sometimes during a drive with navigation, the instructions at times are spoken with the female voice, then sometime later it reverts back to male. I think the female voice happens when there is no network connection, or at least it is when the navigation goes awry such as I don't take the turn suggested. The male returns when things are normal again.

Also, my phone has a built in text to speech facility - it would be cool if Waze used that as it is quite configurable for speaker customization with downloadable alternate voices.

Edit: My phone is a Galaxy Nexus.