can't add music to premiere pro beta

Flip your MP3 files to Wav or AIF.  If you are running Big Sur on an M1 Mac, there are known compatibility issues.You can read all about the current limitations in the top forum post called :  Premiere Pro on Apple Silicon M1.

Adobe is working on bringing full support to the M1 models, but it will take a little longer for the Team to address all the compatbility issues. If you need to operate Premiere without M1 limitations, consider using an Intel based Mac until new fully functioanl versions of Premiere are released to work with the M1 chips.