Presuming they aren't mp3 format files:
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iTunes: How to convert a song to a different file format - http://support.apple.com/kb/HT1550 - including information about different formats and discussion about compression.
In iTunes11: File > Create New Version
In iTunes 11 'Create...' is in the File menu, not the Advanced menu.
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Topic: Need to convert mp4 to mp3 - Zevoneer's post on converting songs - http://discussions.apple.com/message.jspa?messageID=9244740
Converting will place duplicates in iTunes but if you look at the kind you will see one version is mp3. Drag those files to the USB drive.
Feb 17, 2014 11:05 AM