Is the SVN 4771 build you have newer than the recent Dolphin 2.0 release?
If not use Dolphin 2.0, I have that and its playing many games that never used to work such as F-Zero GX perfectly.
My gaming PC's motherboard has faild so I carnt actually check my Dolphin config at the moment, im 99% sure you dont need a Bios though. From what i remember you dont even need the masterkey.bin anymore to play Wii games.
If you really did make the backup your self (unlikley if you carnt be botherd to hook your cube up

) it could be a bad rip, if you downloaded it you also have no way of knowing if the rip is good or not.
This guide shows how i ripped every Gamecube game i own perfectly first time.