check if the mp3 plays fast on the pc too (seriously, heh, sometimes it happens).
bad re-encodes can do it too, I'm not sure if it's a irshell bug but if you have re-encoded every mp3 to the same settings that is ok on the pc but fast on the psp, then, well, the mp3 decoder used in irshell must have some incompatibility.
if that's the case, give us ur mp3 settings (right click, properties, config, advanced or something like that) (or give us a sample mp3)