Firebase getDownloadUrl cannot execute onSuccess or onFailure

I am trying to get URLaudio files in Cloud Storage. I can post it to storage, but this function just returns null. When debugging, it skips onSuccessboth and onFailure.

I know the filename variable is correct, but also tried hardcoding it without success. I mStorageRefpassed StorageReference. fileUrl is currently a global string.

mStorageRef = FirebaseStorage.getInstance().getReference();

String getFileUrl(String filename, StorageReference storageRef) {

        storageRef.child("music").child(filename).getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
            @Override
            public void onSuccess(Uri uri) {
                fileUrl = uri.toString();
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                fileUrl = e.getMessage();
            }
        });

        return fileUrl;
    }

These files are located in a top-level folder called music.