When exporting as .m4a for the first time, Audacity prompts to locate the FFMPEG dll. I clicked the download button, installed FFMPEG, and then back in Audacity clicked Browse. This Browse prompt, by default, is looking for “Only avformat.dll.” Since FFMPEG names the file avformat-55.dll (or whatever the current version is), the file won’t appear until the drop-down is changed to either “Dynamically Linked Libraries (.dll)” or “All files.”
I think it would make more sense and be more intuitive for Audacity to have “Dynamically Linked Libraries (.dll)” be the default, or even to change how it searches for the name of the file so that by default it would appear when viewing the correct folder.
The current Audacity 2.4.2 should automatically detect “FFmpeg for Audacity” when installed.
Audacity 2.4.2 is available via the Audacity website: https://www.audacityteam.org/download/windows/
Installation instructions for “FFmpeg for Audacity”: https://manual.audacityteam.org/man/installing_ffmpeg_for_windows.html