I would first be sure the Sound Preferences are correct:
- Open System Preferences from the Apple () menu.
- Choose Sound from the View menu.
- Click the Output tab to select output devices or the Input tab to select input devices.
- In the "Choose a device for sound output/input" area, select the device by clicking on it in the list. The selected device becomes highlighted.
Connecting headphones or external speakers to the front headphone jack automatically redirects audio to that port when you select either the internal speaker or the speakers of a 24-inch LED Cinema Display. When you unplug headphones or external speakers from the headphone jack, audio redirects back to the internal speaker or the speakers of a 24-inch LED Cinema Display. If the analog audio line-out port, optical digital audio out port, or USB and FireWire devices are selected, connecting headphones or external speakers to the front headphone jack will not redirect the audio.
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