In Audacity 1.3.9 there is support for the LAME MP3 encoding (which allows you to export audio tracks in MP3 format) and the FFmpeg import/export library. The latter allows Audacity to import and export additional audio formats, such as AC3, AMR(NB), M4A and WMA. There is also some limited support for importing audio from some video file formats. However, much of this is still in beta form, which means that not all of the features may work fully.
Libraries provide different features than plug-ins—and, for the most part, allow for additional importing and exporting options. Thus, the features are accessed most often through the File menu's Import or Export options.