packMP3 is a compression program specially designed for further compression of MPEG-1 Audio Layer III audio files without causing any further loss. It works the same as my more popular other compression software packJPG and typically reduces the file size of a MP3 file by 16% based on tests with 6000 randomly selected files.
Download it from here:
packMP3 v1.0g (LGPL)
packMP3 includes source code and is open source under the GPL.
There are no known bugs for packMP3, meaning that it can be considered stable software. Always be careful, though, and never delete any original files without at least having verified the compressed files’ integrity (option -v).
There’s also a third party graphical user interface for packMP3:
packMP3 Options by Michael Lee