[creedo]

mp4 is a container format that can hold almost any combination of audio and video streams. these can be encoded with different codecs. so a .mp4 can have divx video mixed with mp3 audio, or h264 video and aac audio, or mpeg2 video and audio. mkv is similar.

basically your player might be able to handle one kind of mp4 but not another. dunno how psp mp4's are encoded but if core and wmp both fail to play it, you're screwed. better find a good fast conversion program.