In reference to your Mic/Line question: It selects between different input levels/impedences (-10/+4dB), so when it is unselected your input is set up for Mic inputs, when you push the switch it changes to a line level input for recording from things like CD players or Samplers. Hopefully that helps with that.
In reference to your 5.1 speaker setup: Currently each of your 3 cables carry 2 audio signals each (therefore 6 channels). What your firewire interface gives you is 6 MONO outputs so you need new cables to connect it. Depending what input your sub/amp has, will tell you what cables you need.