If your Streamlabs "Theme" did not add your audio source automatically to your Streamlabs main screen, you might need to add it manually.
1. Depending on the Streamlabs "Theme" you're currently using, identify where the "Sources" panel is.
2. Click on the "Plus +" icon next to the "Sources" panel.
3. Streamlabs will ask you what kind of source you want to add - you're looking to add an "Audio Input Device", such as an external microphone or the one internal to your computer. Select it in the list and click "Add Source" near the lower-right hand corner of that screen.
4. The next screen will ask you what name you want to give to this microphone. Choose a name that makes sense for you.
5. The next screen will ask you what is the exact device that you'll be using for this Streamlabs audio source. The drop-down box will list all devices that your operating system has found - make sure to select which one you want to use.
6. You can close this window and monitor your audio in the Streamlabs "Mixer" tabs near the lower-right corner of the screen.