Ubuntu / Debian based packages are often built with dynamic loading disabled. When you build Audacity from our source code, it should link to the installed FFmpeg headers. If required, you can also configure to enable or disable particular encoding/decoding libraries. Configure the build with -enable -shared so that it builds the necessary shared object library (.so) files.

To build libav, obtain the source code from the libav project. To build FFmpeg, obtain the source code from the FFmpeg project. It is recommended to use a shared build, but Audacity will also work with a static build. A static build consists of a single libavformat library, whereas a shared build has at least three libraries (libavformat, libavcodec and libavutil). If you download a pre-built library, this might be either a 'shared' or 'static' build. You may download (or compile) and install a compatible version of the FFmpeg or libav library for your purposes then locate it in Libraries Preferences.

In this case, no additional installation is required, Some distributions (including Ubuntu), include FFmpeg by default when Audacity is installed from the official distribution repository.

Audacity windows installer