I can't remember how I found the schematics. Just a bit of creative googling, I guess. Also, I don't remember what prompted me to suspect the main IC to be an Amtel part (or clone), but something I ran across in my searches, must have suggested it.
Sound quality is fine except for some bass roll off starting at about 120 Hz. It was obviously intended to drive tiny speakers, and they didn't want it to rattle the cones. However, if you use the line outputs followed by the preamp circuit that I included in the notes, and using that to drive another amplifier, then the audio is pretty good. Considering that the player has a number of different equalization settings, it's a shame they didn't provide one that is flat. I should also point out that my preamp circuit includes a 560 pF capacitor which is there to compensate for what appeared at the time to be an overly hot high end. I later concluded that it may not have been the fault of the mp3 player, but rather the amplifier that I had connected to it. So the 560 pF capacitor could be omitted.