您可能在主题的header.php. 加载此文件时$post 还不可用（将在循环中）。所以你需要先申报global $post;.

但这不是最好的办法。如果你交换主题，什么？再来一次
请参见：Where to put my code: plugin or functions.php?

使用以下插件，我们将wp_head 并根据一些Conditional Tags:

<?php
/* Plugin Name: Print Audio OG */

add_action( \'wp_head\', \'print_audio_og_wpse_99152\', 0 );

function print_audio_og_wpse_99152()
{
    // Print only in single posts or pages
    if( is_single() || is_page() )
    {
        global $post;
        $meta_value = get_post_meta( $post->ID, \'meta_audio_attachment\', true );
        if( $meta_value )
            echo "
<meta property=\'og:audio\' content=\'$meta_value\' />
<meta property=\'og:audio:title\' content=\'$post->post_title\' />
<meta property=\'og:audio:artist\' content=\'Test\' />
<meta property=\'og:audio:album\' content=\'Test Album\' />
<meta property=\'og:audio:type\' content=\'application/mp3\' />
";
    }
}