I downloaded the manual for the MacBook Pro and it says the following:But there is only ONE Combined in-/output on my MB.
Connecting Speakers and Other Audio Devices Your MacBook Pro comes with two built-in speakers, a built-in microphone, an [B]audio line in[/B]/optical digital audio in port, and a [B]headphone[/B]/optical digital audio out (f) port. [B]You can connect external speakers, headphones, 5.1 surround systems, and other sound output devices to the headphone (f) port[/B]. The headphone port is also a S/PDIF stereo 3.5 mini-phono jack. When devices are plugged in, sound won’t come through your built-in speakers. Unplug any headphones or speakers to hear the computer speakers. You can record sound onto your hard disk using the built-in microphone located under the left speaker grille. You can also connect external microphones or other audio equipment to the audio line in port. The audio line in port is also a S/PDIF stereo 3.5 mini-phono jack