Whenever you set a text to a slide, you can transform that text to speech.
When you right click on the slide and click on TEXT you can hear a preview on how the voice will sound. You can select the voice language/gender and play the preview.
Please note that this is just a preview, and the final voice won't be the one that you set here.
VSB Pro supports only 1 voice in a video, and the final voice used will be the one you set in Create Video Window.