您可以使用oembed_result. 现在测试URL的HTTP主机www.youtube.com 并添加参数。

oEmbed结果缓存在post-meta字段中，以避免请求过多。为了更新旧帖子，我添加了一个激活助手，它可以清除那些仅用于Youtube嵌入的缓存内容。

<?php  # -*- coding: utf-8 -*-
/* Plugin Name: Add \'wmode\' to video embeds */

register_activation_hook( __FILE__, \'t5_clear_oembed_cache\' );
add_filter( \'oembed_result\', \'t5_oembed_wmode\', 10, 2 );

/**
 * Add "wmode=transparent" query string to youtube embeds.
 *
 * @wp-hook oembed_result
 * @param   string $html
 * @param   string $url
 * @return  string
 */
function t5_oembed_wmode( $html, $url )
{
    if ( \'www.youtube.com\' !== parse_url( $url, PHP_URL_HOST ) )
        return $html;

    return str_replace( \'=oembed\', \'=oembed&amp;wmode=transparent\', $html );
}

/**
 * Clear oEmbed cache for all youtube embeds.
 *
 * @return void
 */
function t5_clear_oembed_cache()
{
    global $wpdb;

    $posts = $wpdb->get_results(
        "SELECT post_id, meta_key
            FROM  `$wpdb->postmeta`
            WHERE  `meta_key` LIKE  \'_oembed%\'
                AND `meta_value` LIKE  \'%youtube%\'"
    );

    if ( ! $posts )
        return;

    /*
    return print \'<pre>$posts = \' . htmlspecialchars( print_r( $posts, TRUE ), ENT_QUOTES, \'utf-8\', FALSE ) . "</pre>\\n";
    /*/
    foreach ( $posts as $post )
        delete_post_meta( $post->post_id, $post->meta_key );
    /**/
}