Preview is an app on macOS for viewing and editing images and PDFs.

According to the Audacity manual it supports MP3. But you need to install additional software for AAC (M4A).

manual.audacityteam.org/man/importing_audio.html

So it’s strange that you are struggling with this. It seems something more is going on here…

Also, just in case it was confusing: M4A is just a file extension used to indicate AAC audio in an MP4 container.

That’s a bummer that AudioTimeliner is struggling with MP3 files. Small independent apps like this usually depend on outside libraries to play back audio. And looking at the version history, it looks like the author has had to make multiple updates to fix playback support over the years.

I see that AudioTimeliner is niche software that has been around for about 22 years, and it’s cross platform. It seems normal to me that it would be picky. Audacity on the other hand, something weird is going on.

I work in higher education, so I understand how relying on niche software like AudioTimeliner goes. I’m sympathetic.

But there is a lack of precision in what you are describing, and your symptoms are directly counter to the Audacity documentation.