Ultra.
With the release of Streamlabs Desktop for Mac, a common question is “Why can’t I capture desktop audio?”. Unlike Windows, macOS does not have any system-level audio routing. This means that individual apps can’t access audio signal from other apps out of the box. However, there are third party solutions that can accomplish this.
As of macOS 11 Big Sur, we recommend BlackHole by Existential Audio. This guide will cover how to install and set up BlackHole for use with Streamlabs Desktop.
Step 1
Download and install BlackHole.
Click here for BlackHole’s Github page, where you can find instructions on how to download the installer. Once you have the .pkg file downloaded, open it and you should see this: 
Follow the instructions to finish installing, entering your password when prompted.
Step 2
Setting up a Multi-Output device
There are several different ways to utilize the features that BlackHole offers, however for the scope of this article and Streamlabs Desktop in particular, we’ll only cover setting up a Multi-Output device so that you can capture and monitor desktop audio.
- Open the “Audio MIDI Setup” utility
- Click the Plus (+) button in the bottom left corner and click Create Multi-Output Device
- Check the box next to BlackHole, the Built-in Output, and any other devices you want to use for monitoring. Important: the built-in output needs to be ABOVE BlackHole in the list. If it’s not, uncheck and re-check the box under “Use” to alter the order.
- Right-click on the Multi-Output Device and select “Use This Device For Sound Output”. Now it’s set as your default output device! All desktop audio will now be output to both your built-in device as well as the virtual BlackHole device.
Now open Streamlabs Desktop and make sure that BlackHole is selected as one of your Mic/Auxiliary devices in Settings > Audio. 
You’re done! For additional questions or troubleshooting, please refer to BlackHole’s Github FAQ and Wiki