This only works if you have two Sims, one that is male and one that is female. Have the Sims each in separate rooms that are far enough away from each other so that you cannot hear the other if you have one on the screen. Have both Sims start practicing speech in a mirror. While watching one Sim, click on the face picture of the second Sim. You will see the second Sim talking, however if it is the male it will have a female voice, and vice versa.