How can I control both monitors’ volume with a Multi-Output Device?

Match the Multi-Output Device to one display in BetterDisplay, then synchronize that display's volume changes to the other monitor. Each monitor must already have working volume control in BetterDisplay. This setup does not create hardware volume support.

  1. Keep your Multi-Output Device selected as the Mac's sound output.
  2. For one monitor, open Settings > Displays > [display] > Device Control > Volume Control…. For hardware volume control, leave Associate this display with an audio device that has native volume controls off and set Matching audio device name to the Multi-Output Device's name.
  3. Under Settings > Keyboard > Volume Control Affects:, choose Display with the same name as current audio device. Confirm that the matched display allows keyboard volume control.
  4. Put both displays in an active group. Under Settings > Groups > [group] > Synchronization Settings, enable group synchronization, choose Add New Synchronization… > Volume, and allow the matched display to be a source and the other display to receive its changes. Volume synchronization requires Pro.

Test at a comfortable listening level. The name override makes the keyboard target the display's existing controller. It does not adjust a master mixer inside the Multi-Output Device. Synchronization then sends the adjustment to the other display's controller.

By default, Delta syncing preserves the difference between their starting volume levels. Turn it off if you want absolute values to follow instead. Equal percentages do not guarantee equal loudness from different speakers. Add a separate Mute synchronization entry if both monitors support it and you want their mute state linked too.

See Volume Control and Synchronization Settings for name matching and source/target options.

For a speaker that uses native macOS volume control, you can instead associate its display with the individual audio device under Volume Control…. Enable Associate this display with an audio device that has native volume controls and select the component device that actually has adjustable volume. Associating the non-adjustable Multi-Output Device itself does not create a master volume control.

Once the individual controller works, that display can receive group volume synchronization too. Keep the keyboard source configured for the device you intend to adjust.