This is my third day with my MP4 player.
I've open the player and i've found 2 chips. 1 samsung memory and a atj2085tj chip. It works with Z80 intruction set. Also searching for the datasheet of the chip, i've found a page that is working on a proyect for making a firmware, an atj2085tj emulator and some interesting things. Here is:
http://www.s1mp3.org/