um, yes you CAN compress mp3's... DietMP3 would be an example... also, another good file compression utility (straying away from mp3) is UHARC.
just added this in: you can also compress .jpg files, although its a little different. Its more info used by advanced image editors not needed by standard image viewers, so its not the image you are compressing, but just getting rid of uneeded fluff in the image.
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