Hi - There is an issue importing sound files in 21.0.8 which we have fixed internally and validating it. Will let you know as soon as it is ready. Apologies for the inconvenience.
This 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.
Make sure you are using a mp3 or .wav. Those are the best audio files to use.