Mp3Wrap is a command-line utility that wraps two or more mp3 files in one
single large playable mp3, without losing filenames and ID3 informations (and
without need of decoding/encoding). Also with the possibility of including
other non mp3 files, such as PlayLists, info files, cover images, inside the
mp3. 

WWW: http://mp3wrap.sourceforge.net/