FAQ:Installing the FFmpeg Import/Export Library
From Audacity Development Manual
The optional FFmpeg library allows Audacity to import and export a much larger range of audio formats including M4A (AAC), AC3 and WMA and also to import audio from most video files.
Because of software patents, Audacity cannot include the FFmpeg software or distribute it from its own websites. Instead, following the links below to instructions to download and install the free and recommended FFmpeg third-party library.
Please see Installing FFmpeg on the Audacity Support site for FFmpeg installation details.