Did you format the sd card on your phone? Create a folder on your PC, to store the audio files on your phone, and transfer them off the card. Do a full format of the card on your phone. After this, I usually dismount the card on my Note9, then remount it. Once you have finished, transfer the audio files back to your card. Again, the way I transfer my audio files is to connect the phone to the PC, using the supplied USB cable. After that, I open FILE EXPLORER on the PC, browse to the file folder containing the music, then drag them onto the card in the phone. Hopefully, this solves your issue.