Similar problem here, after updating from Audacity 2.1.1 to to 2.3.0 on a Mac with macOS 10.12.6 Sierra:
Trying to open an m4a file results in an error message saying “This version of Audacity was compiled without Quicktime support” (or so).
Let me point out, that the ffmpeg library was not present on the machine, as Audacity 2.2.1 used to be able to open m4a without it.
As suggested above, I deleted (actually: renamed) ~/Library/Application Support/audacity/ and started Audacity again: A new directory was created, but Audacity would still run into the same error, so I restored my previous configuration directory.
Assuming that support for AAC and ALAC is indeed missing, I installed the ffmeg library. As a result, m4a files (AAC as well as ALAC) can be opened now.
I have no problem with this solution, but it should be clearly documented, and I’d wish “native” m4a support being provided again with the next release. After all, AAC and ALAC files are very popular on the Mac platform.
PS:
Could this be the same bug that had happened with 2.1.0 ? https://forum.audacityteam.org/t/audacity-2-1-0-ffmpeg-required-for-mp4-m4a-mov-alac-import/38209/1
So, why were some users seemingly able to fix this by just generating a new config file? I suppose, they just fixed a broken path to the ffmpeg library, which they had installed. However, the previous version of Audacity would import m4a even without the library.