dottie-o: that's not an audio input jack. The QuickSilver models only support audio-in via USB devices, like USB microphones and such.
The port you think is an audio-in port is most likely the audio-out port for the Apple Pro Speakers. They had a special plug that accepted high-power signals from that port, so you didn't need to plug the speakers into a standard wall jack, like most speaker sets.
It's not for audio-in. Your only option for audio-in on the QuickSilver is a USB device, or a sound card... but I cannot recommend anything but the highest-end sound cards (like the M-Audio cards) as the drivers for retail-level audio cards just are plain bad (IMO).