iTunes can read MP3 and AAC formats. But if you bought the tunes from the iTunes store, they will be PROTECTED AAC (which you can't convert to MP3 with iTunes).

Look inside the iTunes Music folder for the actual files, and see what kind of file they are...

If they are not protected, Quicktime will convert them to MP3 for you, at least.
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An arranger is just a tool. What matters is what you build with it..!