Kbase Article of the Week: LED Cinema Display (27-inch), LED Cinema Display (24-inch, Late 2008): About Headphone Sound Redirection →

Apple Support:

In certain situations, when you plug headphones into a Mac connected to an LED Cinema Display (24-inch, Late 2008) or LED Cinema Display (27-inch) then select the display’s speakers in the Output tab of the Sound pane of System Preferences, your selection may revert back immediately to the headphone choice. When you plug iPhone headphones into a Mac connected to an LED Cinema Display (24-inch, Late 2008) or LED Cinema Display (27-inch) then select the display’s microphone in the Input tab of the Sound preferences, your selection may immediately revert back to the iPhone headphones choice.

To select the display’s sound output, first unplug the headphones, iPhone headphones, or external analog speakers.

To select the display’s built-in microphone, first unplug the iPhone headphones.