Mac Pro (Early 2009 through Mid 2012): Audio may not switch when new devices are connected
Audio devices may behave differently on Mac Pro (Early 2009 through Mid 2012) than on other Mac computers.
Mac Pro (Early 2009 through Mid 2012) implements discrete audio engines for audio processing.
When using either the Mac Pro computer's analog audio line-out port, optical digital out port, or external devices such as USB Speakers or USB/FireWire audio interfaces, you need to switch manually to the appropriate device to enable its operation:
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.