Audacity version 2.4.2, (late 2020), Audacity became very fussy about what type of mp3 was acceptable … Version 2.4.2, Opening an mp3 file for editing - something about FFmpeg [SOLVED] - #25 by steve
A workaround could be to use Audacity’s free competitor OCEN audio which is more tolerant, and could be used to convert the mp3s that Audacity will not play into a form acceptable by Audcaity, e.g. WAV or FLAC.