I think that is very weird that Mac mini doesn't have a mic or a simple line-in interface, i had to wait for 15 days and pay for the iMic 53 €. For those who doesn't know what it is:
The iMic universal audio adapter is a USB device that adds stereo input and output to your Mac or PC. Connect virtually any microphone or sound input device to your iBook, PowerBook, PowerMac or other Mac or PC with a USB port. Yes, iMic supports both Mic level and line level input. It also supports line level output for connecting speakers or an external recording device.