USB surround sound cards are ten a penny these days, but Firewire ones are a little more unusual. The Griffin FireWave is one such beast: it connects to a Mac via the Firewire connection, and includes a pass-through port so you can still connect your iPod or digital camcorder. The $100 device can drive a 5.1 speaker set, so it's a good way to add surround sound to a Mac mini.
Griffin Firewave Krunker
FireWave [Griffin Technology]