Headers already sent warning in Admin source is media.php 3140

Looking at the docs for register_taxonomy, it takes 3 parameters, the 3rd of which is the arguments, and it looks like in your code you have the arguments in the second parameter. Perhaps this is one of those tricky bugs where this was somehow working in previous versions and it officially shouldn’t have been.

If songs is the object, you need to add another parameter for the taxonomy key, e.g.:

    register_taxonomy(
        'songs_tax',
        'songs',
        array(
            'label' => __( 'Songs_Tax' ),
            'sort' => true,
            'args' => array( 'orderby' => 'term_order' ),
        )
    );