Well, you could....
1. get an RCA to Mini adapter
2. Plug the Gamecube into the Adapter, the adapter into your mic port.
3. In the OS 9 (seems OS X don't have the feature) In the sound control panel, select input then sound in, and play input though speakers (or something like that I forget how its worded.).
I have done this to get My radio, and the sound from a game (myth) coming outta the same speakers

Never say never!!!
-Owl