use supproted file formats, bitrates etc, How to use sound in Adobe Animate
However, mp3 sound data is compressed and smaller than WAV or AIFF sound data. Generally, when using WAV or AIFF files, it's best to use 16-22 kHz mono sounds (stereo uses twice as much data as mono), but Animate can import either 8- or 16-bit sounds at sample rates of 11, 22, or 44 kHz
This was a known 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.
note: adobe updates get rolled-out to users at different times over a period of several days. if you don't see an update in your cc app, wait. it will appear within a few days.