Data is usually stored efficiently in mp3 files. Since mp3 files are already compressed, they offer a good size to quality ratio usually when it comes to music files.
This means that it does not make much sense to compress mp3 files further at least if size reduction is the desired result.
Compressing a standard mp3 file will usually result in a size reduction of a few Kilobytes which, even if applied to a collection of thousands of mp3 files, does not free up more than a few Megabytes of disk space. High bitrate mp3s on the other hand are not that efficient and it is possible to reduce their size by about 2-10%.
While that may appeal to you, it may result in a quality reduction of the audio file so keep that in mind.
Mp3 Packer is a tool that has been specifically designed for the purpose of reducing the size of mp3 files. It can be used to process any number of mp3 files since it can process all files in a directory and even its subdirectories at once. The size reduction will not affect the quality of the mp3 files, a pure lossless size reduction so to speak.
Various output settings are available that have an impact on the newly generated files. It is possible to specify a minimum bitrate, select the output type (vbr or cbr), strip non-mp3 data from the start or end of the file, append text to the filenames, alternate broken frame behavior or super-squeeze files.
The mp3 packer will process all audio files in the selected directory and display information about the size gain at the end.
Another interesting option of Mp3 Packer is the ability to create profiles. This can be useful to make sure that the mp3 files will always be packed using the exact same settings. MP3 Packer is a portable application which means it can be run without installation.