To set your input device as StageSound in Streamlabs ensure StageSound is opened and is turned on. In Streamlabs add a new Source by clicking the plus in the sources section.
Select "Audio Input Capture" then click "Add Source" in the bottom right corner of the window.
Add a name for the source, we recommend naming it StageSound, then press the "Add Source" button. In future StageSound will appear under Add Existing Source with whatever name you choose.
If you already have an audio input capture device added, then make sure to click "add new source instead" located in the bottom left corner to get to this menu.
Select StageSound from the Device dropdown menu then press the "Done" button.
StageSound will now be added as a source within your current Scene.