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I have a dual monitor setup (iMac + Thunderbolt Display). The System Preferences > Sound > Output setup only allows me to choose between one or the other monitor. This causes the sound to come only from left or right side.

Any way to get both monitors to play audio simultaneously?

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up vote 5 down vote accepted

You should be able to use the Audio MIDI Utility (a.k.a. Audio MIDI Setup on Mountain Lion) to create an Aggregate device to mix both output destinations into one item the OS can select.

The Help topics in OSX around this area are...

Play audio through multiple output devices at once

Combine audio devices to use a single device

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I found this as well, but StackEx didn't let me answer my own question. ;) "Multi-Output Device" worked. Only problem is now the master volume buttons (F10, F11, F12) are disabled. If I need to change the volume or mute, I have to do it on the app (which is not always available) or go back into Audio Midi and change volume for each speaker. (Try muting really fast if the phone rings...). Any ideas? – Luciano Mar 4 '12 at 21:04
I like the TakeFive app to rapidly pause iTunes for a phone call - not sure if it controls your player of choice, but it's awesome from the Iconfactory - on the app store too... – bmike Mar 4 '12 at 21:20
@Luciano Can you post more information about the exact settings? I tried using The Audio MIDI Utility, but I could not seem to figure out the right settings. – daviesgeek Oct 29 '12 at 18:21
This works, however as the OP said you can't use the volume buttons. You have to go back to the MIDI settings and change the volume for both displays. – jsheffers Jan 21 '13 at 18:21

Yes, it's possible.

By creating an aggregate device, you can combine the audio interfaces of the dual Apple LED Cinema Displays into a single speaker.

Here's an overview of the process:

  1. Open /Applications/Utilities/Audio MIDI Setup
  2. Click + to add a new Multiple Output Device
  3. Select one or more Display Audio devices
  4. Adjust the volume slider for each Apple LED Cinema Display

Combining two Apple Displays for Audio Sound Output

There is one caveat to this setup — you can't adjust the volume using either the keyboard shortcut or the speaker icon, as it appears disabled in the OS X Menu Bar. you can Option + Click to toggle between audio output devices though.

Sunflower is an open source third-party app that will give you the volume control back, but since it's a kernel extension you should understand the implications before installing it.

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