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If you are asking about the file storage format, it really varies. Some flash based players use a FAT16 format just like a hard drive. Others, especially older ones, often use a proprietary format that is unique to that manufacturer. This is why the MP3 player can write an removeable flash card, but when you take the flash card to a separate PC based card reader it appears blank or full of gibberish. The PC based card reader is expected FAT16.
There was about a year ago, a project to transform the $20 AVR Butterfly into an MP3 player with an MMC card and an external board that had the MP3 decoder. Search the web or www.avrfreaks.net and you will find it.