If your reciever is capable of Dolby Prologic 2 (DPL2) you have to set the mode for it to DPL2 movie mode for the sound to be correct. The gamecube will just produce dpl2 when it is set to do so in game.
DPL2 is just a way of putting information for the other surround channels on top of the channels from the GC, so long as the stereo rca plugs are connected to the reciever (i'm not sure which of those you've got) once that input is selected (on the reciever) you should get sound out. Just set the reciever to decode pro logic 2 and it *should* just work.
GL