On macOS, you can create a Multi-output audio device, or an aggregate audio device and route audio to multiple devices at once. It’s fairly easy to do once you understand the concept behind it. It does come with a shortcoming; no volume control. When you configure a multi-output device, the volume controls suddenly become inactive. Tapping the volume keys on a MacBook will show the volume control with a cancel button. The Sound preference in System Preferences will likewise show a volume slider that is inactive. Here’s how you can mange the volume of multi-output devices on macOS.
Volume for multi-output devices
You will need to go through the Audio MIDI app to change the volume for multi-output devices. Open the app and select the device from the column on the left. Check if all the devices that have been added to the multi-output device are listed under it.
In the screenshot below, two different devices have been added to the multi-output device but only one appears listed under it. This isn’t a problem but it will change how the volume is managed to a minor extent.
The first audio device that’s a part of this multi-output device is the built-in speakers and they’re not showing up under the device. In such a case, select the device from the column on the left and use its volume control to change the volume.
Next, move on to the other device(s) listed under the multi-output device. Use the volume sliders in the pane on the right to change its volume. The volume for the multi-output device will now reflect the new levels you’ve set.
Change master device volume
There is one other way to change the volume of a multi-output device however, this changes the volume of the device as a whole and does nothing to the individual volume levels of the various devices. It may result in the devices being more quieter than they should be.
Determine which audio device is set at the Master device. You can check this from the Audio MIDI app. Select the multi-output device and look at the Master Device dropdown.
Open the System Preferences and go to the Sound preference. Select the Output tab, and select the device that is used as the Master device in the multi-output audio device. Once it’s been selected, use the volume slider at the bottom to change its volume. On this same tab, you will see the multi-output device you created. After you’ve changed the volume for the master device, select the multi-output device. Play audio on your Mac and the change in volume will be reflected in the multi-output device.
Source = “https://www.addictivetips.com/mac-os/manage-volume-for-multi-output-audio-devices-on-macos/”