m4p WHAT IS THIS

George Hyde's reply is correct for MPEG-4 files with an .m4a extension.
MPEG-4 audio files can have several extensions.
.m4a files can be AAC and Apple Lossless.
.m4p files are Protected AAC files purchased at the iTMS.
They cannot be converted into another format (with iTunes or another application), due to the protection.
They cannot be burned to Audio CD, except with iTunes.
Conversion of non-.m4p files to other file types can be done easily in iTunes.

Are your songs purchased at the iTMS or did you batch-convert the extension with some some script?

M

17' iMac fp 800 MHz 768 MB RAM Mac OS X (10.4.6) Several ext. HD (backup and data)

An m4p file is actually Apple's AAC format. It is not the same as an mp3 file in the respect that it allows higher quality in a much less lossless compression as opposed to mp3 and I believe if I'm not mistaken multi-layer audio much the same as a native QuickTime files as well.

To convert it over to mp3 you will need an audio editing program such as Sound Studio 3 or some equivalent. To keep all future audio imports from being saved in iTunes as m4p (AAC) files, go to the iTunes preferences and change the import options.

Hope this helps!

Note that as long as your AAC files are not protected, you could convert them to mp3 within iTunes simply by choosing "convert selection to mp3" in the Advanced menu.