“*.m4a is an Advanced Audio Coding file. Without the optional FFmpeg library, Audacity cannot open this type of file. Otherwise, you need to convert it to a supported audio format, such as WAV or AIFF.”Audacity
Audacity natively supports audio files in WAV, AIFF, Ogg Vorbis, FLAC, MP2 and MP3 formats. M4A, AAC or other audio containers using the Apple Lossless Audio Codec are not supported by this application.
However you can install the FFmpeg Library to enable Audacity to support more common formats such as AC3, AMR(NB), M4A, MP4, WMA, etc.
There are different resources or websites you can download the FFmpeg library. Following are some of the websites you can get the installer.
Once you finished the installation of FFmpeg on your computer, restart Audacity, Audacity should automatically detect FFmpeg and allow you to use it. Otherwise, you need to click Edit from the top menu bar of Audacity and choose Preferences from the drop-down menu. The Preferences opens in a new window. Choose Libraries from the left panel, you can then find the options to download FFmpeg or manually locate FFmpeg library.
Audacity needs the file ‘avformat.dll’ to import and export audio via FFmpeg. Browse to the location of this dll file on your PC so Audacity can use it.
In this dialog window, click Browse… to locate the avformat-*.dll from the FFmpeg folder you downloaded/installed on your computer. Once you’ve found it, click Open, then click OK twice to close the preferences.