just played a little bit with different cables and different sound-systems and then realized something:

i was able to play in 60Hz with my PAL-Gamecube without an RGB-Cable, EVERYONE says u need a scart cable but u don't just use the av cable with the scart adapter and it will work fine (yes i tested if its really 60Hz, you can clearly see it in games like virtua striker 3)

but what i couldn't get workin was:

connecting my gamecube to the video in of my dvd-player and let the sound play over my 5.1 system, it was only stereo and only two speaker, is there any way to make it run over all speakers? doesn't have to be surround sound, but at least stereo over all speakers would be cool