That wall plate looks like it has spring terminals and you press the red and white buttons on it it to open the little silver hole above and insert the wire. The only requirement for speaker wire you'd have is to make sure the gauge of wire you use isn't too thick to fit in the little holes. If you want to stick with that plate, you could terminate your speaker wires with pin connectors and insert the pins into the plate instead of the bare wire.
You could also swap those plates for a wall plate like this:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=261-354 which is what I use in my house. There will be no performance difference but they look nice and you can terminate your speaker wires with banana plugs for easy connection and disconnection.
It's possible the speaker wire in the wall that is connected to the wall plate runs to the speakers on the outside but if the house was pre-wired it is also possible that the speaker wires run to some other location in the living room where there is a Volume Control (or at least space for it if it does not exist yet) and the wires from the VC location actually go to the speakers on the outside.
You can't connect an iPod directly to the speaker wall plate. You connect the iPod to a receiver and speaker wires from the receiver to the wall plate. The intent of speaker wire wall plates is simply to hide long runs of wire behind the walls instead of having it visible on the outside running across the floor or up the walls.
