You cannot put the supercard SD into the GBA and play videos or music for obvious technical reasons.
The GBA has an ARM7, which works at 33Mhz. The NDS has an ARM7 and ARM9, which together have 99Mhz. The NDS cannot play all popular music formats (can play MP3s well, though) and the NDS can only play one type of video format, because it takes advantage of all hardware acclerations the DS can give. (anyways, the DS can only give at max 10fps for playing back video)
It's hard te believe that it is completely impossible to play movies on a GBA, because they advertise that all the time with the different cards and you've also got products like the GBA Movie Player which plays movies on a GBA. SO I really hope that that's not the issue...
how can i play videos on SuperCard? i'll try Movie Converter v.1.22 & v.1.30 but in the end of converting it crashes. then when i try play it on GBA through FilmPlay.gba i saw just colour cubes. how can it be done?