In order to mix your audio effectively, you need to be able to hear it. Start by plugging headphones or speakers into the headphone jack on your system.
You may not hear any audio right away, even if you see audio levels. If this is the case, go to your Windows Control Panel and navigate to your Sound settings. Under the Playback tab settings, look for Speakers - RealTek High Definition Audio. Select that source, click Set as Default, then click OK.
In Livestream Studio, check the headphone volume meter in the upper right corner and adjust the level to the desired volume. You can also adjust your Windows volume as you would when using any other program.
This does not affect audio levels of the inputs, Preview, or Program.
You will know which sources you are hearing in your headphones by checking the Audio Mixer. The bottom of each source has a headphone button.
Any headphone button with a white border is currently being heard in your headphones. You can monitor as many sources at any given time as you need. Click on the headphone button to stop monitoring the corresponding source; you will notice the white border will disappear from the button.
This only impacts your local headphone output and does not stop the audio from outputting to your stream and/or recording.
This also applies to monitoring Program audio, which is typically represented by Record Out / Stream Out.