m4p - protected AAC files from iTunes Music Store
Apple recommends that you protect your investment in m4p files by backing up your library.
AAC files are the naked audio without artwork etc. It makes no sense to drop those on File Juicer.
M4A files are AAC files with cover art and other meta tags.
AAC - Advanced Audio Coding
While it is now mostly known from the iTunes music store AAC is an ISO standard.Mac OS X Welcome Music
If you poke around in the System folder:
Panther:
/System/Library/CoreServices/Setup\ Assistant.app/Contents/Resources/
Tiger:
/System/Library/CoreServices/Setup\ Assistant.app/Contents/Resources/
TransitionSection.bundle/Contents/Resources/
Googling on this I found that the full length versions are:
Jaguar:
Sofa Rockers by Sofa Surfers remixed by Kruder & Dorfmeister, The K&D Sessions Amazon, iTunes Music Store USAPanther:
Eple by Röyksopp iTunes Music Store USA or iTunes Music Store EU?Tiger:
Bytecry, by Weevil iTunes Music Store USA or iTunes Music Store EU?AppleScript and iTunes

AppleScript is a programming language to ask applications to do stuff, and you can do the simple things with "Script Editor" you find under AppleScript in you Applications folder.
Here is a screen shot of how to tell iTunes to get the name of the currently playing Album. You can get much more information about your music this way, and it is with this tool many itunes add ons has been made. To see more about what you can ask iTunes about with Apple Script see the iTunes AppleScript dictionary