@robinp7720 's answer did not work for my solution.

        $output = '<div class="song_list">';
        foreach($catalog as $catalog_item)
        {
            $catalog_item_parts = explode('.', $catalog_item);
            $extension = $catalog_item_parts[1];
            if($extension == "html")
            {
                $catalog_title = $catalog_item_parts[0];
                $catalog_title = str_replace('_', ' ', $catalog_title);
                $catalog_title = ucwords($catalog_title);
                $output = $output . '<a href="../songs/'. $catalog_item . '">&gt;&gt;</a><br>';
                $output = $output . '<iframe src="../songs/'. $catalog_item . '" height="200"></iframe>';
            }
        }
        $output = $output . '</div>';

.main_body .song_list iframe
{
    width: 100%;
    font-size: 2em;
}