in itunes, right click>get info>options>set start and end time of piece you want. THEN rightclick>create mp3 version (or it could say AAF file) and your new file will be only to the time that you set in options. REPEAT steps but setting new start and end times for each new desired piece. but if you have garageband you could also use that by importing your file and chopping it however you like and then publishing as mp3's and exporting them to itunes. your choice.