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(the card was in a port on my PC when i saved the files)
Another avenue to approach, instead of using your PC Memory Slot to directly transfer to the memory card, try to directly transfer them from the USB Port to PSP this may help. If this is to no avail, maybe attempt to reformat you memory card and transfer the mp3's once again. Before doing this backup any gamesaves you might want to hold on to of course. As well try downloading a sample mp3 from an indie bands website or somewhere along the lines first, see if they work. This will narrow down your troubles.