|Probably the most chance for success is to use the MP3 cart hack and FatJBox (I didn't say it was the easiest!). This was the path I took, and I am able to load my own binaries via an SD card. I never got around to figuring out how to program Juicebox sound, and I haven't messed with it in a while.|
So, to play your own MP3's search Ebay, etc. for the MP3 kit - no hacking required.
To play your own videos, do the hack above and write your own video player (or find a way to copy the player off of a juiceware cart).