How do I turn these values from MYSQL into an array

Use $wpdb->get_col. It will return a one-dimensional array that you can use in post__in

$allposts = $wpdb->get_col("SELECT `music_id` FROM `custom_table` ORDER BY id DESC LIMIT 5");

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)