Is it possible to open a M4A file (MP4 audio) with chapters in Audacity on MAC?
If so, how?
I know it’s possible in Windows and Linux by installing the FFMPEG extension.
Audacity does not support Chapter tags on any platform, though it may be possible to write them when exporting files by using the “external program” (https://manual.audacityteam.org/man/exporting_to_an_external_program.html) option, FFmpeg, and appropriate command line switches. If you don’t know what that means, then probably better to use a different program for adding Chapter tags if you need them.
Audacity cannot import chapter tags.
Note that chapter tags are non-standard and are not widely supported. As they are non-standard, there is no guarantee that tags that work in one program will work in another, though tags created by iTunes “should” work in Apple products and “may” work in some other players.