I've tried both mp3 and WAV, both fail. I've ran my files through Media Encoder and switched them to 44.1kHz 64kbps and they still fail. Oddly there are some files that do work for whatever reason (and they happen to be mp3 files). The only pattern I've noticed is that ANY audio file I had in my computer before installing Animate works fine but EVERYTHING I've downloaded since fails no matter what.
I'm attaching 2 files, the first is a file I had in my computer before installing Animate, the second one is an mp3 file I ran through Media Encoder to change it to WAV. The first file I can import fine but the second one (in both the original mp3 and WAV) cannot be imported.
The audio has to be 44100Hz 16-bit PCM WAV to import without issues.
Any other format may be misinterpreted.
ps. It seems that the latest version 21.0.8 has a newly introduced bug/regression with importing audio.
Just try with an older version of the program.
This issue has been addressed in the latest Animate update - 21.0.9 released today. Please update your copy of Animate to the latest using Creative Cloud Desktop application to avail the fix.