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Alternate DVD Music
Instead of using both audio cords (red & white ones), use one of the DVD's Audio cords and one of the GameCube's Audio cords. This way, you can hear the game's music & the DVD's music. But remember, turn the game's music off in the options (if you can), and make sure you have the GameCube's Video Cord in and not the DVD's.
Alternate start-up sound
Power on the GameCube, then hold Z while the GameCube logo is forming to hear a different start-up sound.
Change Screen Refresh Rate
While powering on the system, hold B until the message "Would you like to switch to 50hz mode?" appears. This allows you to switch the screen refresh to match that of PAL games. If playing on a PAL system, the message will be "Would you like to switch to 60hz mode?", allowing the screen refresh to match that of NTSC games.
Control starting logo
While the cube bounces down, hold A to form the "G" logo to spin it.