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");
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");