If these are M4A/MP4 files it’s a bug in Audacity 2.1.0 unfortunately. You can fix it by installing FFmpeg. Please see this topic for download link and more explanation: https://forum.audacityteam.org/t/audacity-2-1-0-ffmpeg-required-for-mp4-m4a-mov-alac-import/38209/1
Gale