I once thought this problem had me beat.... My solution worked for every file I received. Even if your file has a .mp3 extension, try renaming it anyway ie: phatsong.mp3, at this point the file should be converted to a readable mp3. Also, try dragging it onto your mp3 player icon (this works in the case of iTunes) or else try importing or adding your file from you mp3 player. Hope this helps, because it took me a long time to figure it out myself.