I assume ponce is correct but see this also.
Some Linux distributions (for example, Ubuntu) now package Audacity with MP3 encoding and FFmpeg support already linked dynamically to the relevant system libraries. In these packaged builds there is no need to locate LAME or FFmpeg, so Audacity will have no "Libraries" Preferences. Users compiling Audacity from source code must still follow the directions below.
You may download (or build) a suitable FFmpeg library for your own purposes. To build, obtain the source code from the FFmpeg project. If you download a pre-built library, this might be either a "shared" or "static" build. A static build consists of a single libavformat library, whereas a shared build has at least three libraries (libavformat, libavcodec and libavutil). It is recommended to use a shared build, but Audacity will also work with a static build.
If you compile Audacity from source code, it will link to the installed FFmpeg headers, which should always be compatible with an installed FFmpeg binary.
Warning icon FFmpeg version information: Audacity should support building against FFmpeg 0.5 through 0.8. It may not currently be possible to build Audacity against later versions of FFmpeg.
Locating the FFmpeg library manually
If you installed FFmpeg while Audacity was running, then the first time you use FFmpeg to import or export, Audacity will ask you to configure Preferences to locate the FFmpeg library. To do this, access Preferences then click the Locate... button, then Browse....
FFmpeg Find Library and Download buttons
If the "FFmpeg Library Version" says "not found":
Click "Find Library".
If a "Success" message indicates Audacity has now automatically detected the library and asks if you want to detect them manually, click "No", then OK to close Preferences.
If the "Locate FFmpeg" dialog appears, click "Browse...".
Navigate to the avformat library, and select it. It may have different names, such as "avformat-52.dll" on Windows, "libavformat-52.dylib" on Mac or "libavformat.so.52" on Linux.
Click "Open" then "OK", and "OK" again to close Preferences.
"FFmpeg Library Version" should now show a number. If not, you may have installed the wrong libraries. Click the "Download" button to obtain the correct library for your operating system.