I assume you can play the files with other software?
You don’t normally need FFmpeg to open MP3s but sometimes it helps with “imperfect” MP3s.
Audacity is more “picky” about than most software but if you can play the files you can probably convert them to WAV with a 3rd-party conversion program. (I’m a Windows guy so I can’t recommend one… iTunes might be able to convert it to WAV.)
On a technical note, this is a bug in mad (which last got updated in 2004). Audacity 3.2 will switch to mpg123 (last updated 2 days ago) which shouldn’t run into this issue. You can download the current alpha on https://audacityteam.org/nightly and see if it works for you there.
Since you don’t like my easy solution… [u]MP3 Diags[/u] can often diagnose & repair MP3s.
It also wouldn’t hurt to check the files with [u]MediaInfoOnline[/u] to make sure they are really MP3s. That sometimes happens and if it’s actually an AAC file, simply renaming it should solve the problem.
If it’s really an MP3, MediaInfo will report:
…And a bunch of other information. (But it doesn’t seem to find errors.)