To hear audio in your headphones or monitoring speakers while using Livestream Studio - ensure that you have an active audio source in your Audio Mixer and that you have activated the headphones icon button for that audio source.
Next, check that you have set your volume level in the headphones slider bar at the top-right of your Studio interface.
If you are still not hearing audio in your headphone or monitoring speakers, minimize your Studio interface, and go to Start > Control Panel > Sound.
When you are in the Sound window, find the selection for "Speakers - High Definition Audio." In the drop-down menu for "Set Default," select "Default Device."
You should now hear audio in your headphones or monitoring speakers. Close the control panel and go back to your Studio interface.