Multi Output Device

Setup

1. Open Audio Midi Setup

Use Spotlight command+space and type Audio Midi Setup. Click on the Audio Midi Setup app to open.

2. Open Audio Devices Window

If the Audio Devices window does not immediately appear when opening Audio MIDI Setup select Audio Devices from the Windows drop down menu in Audio MIDI Setup.

3. Create Multi-Output Device

In the lower left hand corner of the Audio Devices click the + icon. Select Create Multi-Output Device

4. Select Output Devices

In the new Multi-Output device you just created select the audio devices that you want to output audio. It is highly recommended you enable Drift Correction for all devices except the Clock Source device.

IMPORTANT: Due to issues with macOS the Build-in Output must be enabled and listed as the top device in the Multi-Output

5. Set Audio Output to Multi-Output Device

right-click on your Multi-Output device and select Use This Device For Sound Output. Audio should now be playing through your speakers and through BlackHole.

Troubleshooting

I can't change the volume of a Multi-Output device

Unfortunately macOS does not support changing the volume of a Multi-Output device but you can set the volume of individual devices in Audio MIDI Setup.