I'm afraid voice sets are hard-coded in the game's source. You can only modify that by making your own mod.
I suppose I could add female voices to the Ssrathi and Swarm, but first we would need a female model/avatar for both of those races. Then again, while Ssrathi do indeed have female snakemen (Naga) and reptiles in general have two sexes, I think the Swarm are hermaphrodites (one sex).