LameLib-Carbon fpr OS X

 LAME is a library which allows some programs to encode MP3 files. LAME is free, but in some countries you may need to pay a license fee in order to legally encode MP3 files.Once you have unstuffed the archive, you will have a file called LameLib. To use it with Audacity, you can put LameLib anywhere you want, but the first time you want to export an MP3 file, Audacity will ask you for the location of LameLib, so you'll want to remember where you put it.
http://spaghetticode.org/lame/