With a Mac Mini, you can't. At least under MorphOS that is. You may use an USB based Soundcard, however that will work for OS X/Linux(?) only. So you have to get either a Powermac (with Audio in) or you may do your recordings with another OS and edit it with Audio Evolution afterwards.

Another Option might be a bounty which is appealing enought to get someone working on Poseidon (USB Stack) to get the needed stuff done, to support an USB Soundcard. Allas, since this "Problem" is wellknown for quite a while, I doubt that will happen any time soon.