iTunes music files are protected aac files, you could not use them as a Flash slideshow background music. All you need to do first is to convert the iTunes protected AAC to MP3, then you can use the mp3 file for your flash project.
iTunes protected aac files have .m4p file extensions, so they are called M4P files too. This website site introduces a m4p to mp3 converter software which can be used to convert the iTunes music to MP3.
This is a screenshot from their website:
Actually, this software installs a virtual cd, when you burn your iTunes playlist to this virtual devices, you can get the files unlocked.