尝试以下操作：

在单个视图上，您需要使用get_the_terms() 而不是get_terms(). 这样，您就得到了实际分配给该职位的术语，而不是现有的所有术语。

    $post_type = \'mp3\';
    $tax = \'post_tag\';
    $post_count = 5;
    $counter = 0;
    $tax_terms = get_the_terms($post->ID, $tax);

    if ($tax_terms) {
      foreach ($tax_terms as $tax_term) {
      $args = array(
        \'post_type\' => $post_type,
        "$tax" => $tax_term->slug,
        \'post_status\' => \'publish\',
        \'orderby\' => \'rand\',
        \'posts_per_page\' => 5
      );
      $my_query = null;
      $my_query = new WP_Query($args);
      if( $my_query->have_posts() ) {
        echo \'<ul id="related-posts" class="item-list">\';
        echo \'<h6>Related Posts</h6>\';
        while ($my_query->have_posts() && $counter < $post_count) : $my_query->the_post(); ?>
        <li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
        <?php
        $counter += 1;
        endwhile;
        echo \'</ul>\';
       }
       wp_reset_query();
      }
    }

NOTE: 此代码将生成多个相关帖子列表。如果当前帖子分配了2个标签，您将得到2个相关帖子的列表。如果您希望从两个标签组合中找到一个相关帖子列表，您可以使用以下内容：

$post_type = \'mp3\';
$tax = \'post_tag\';
$post_count = 5;
$counter = 0;
$tax_terms = get_the_terms($post->ID, $tax);

if ($tax_terms) {
  $tax_term_ids = array();
  foreach ($tax_terms as $tax_term) {
    $tax_term_ids[] = $tax_term->term_id;
  }

  $args = array(
    \'post_type\' => $post_type,
    "tax_query" => array(
      array(
        \'taxonomy\' => $tax,
        \'terms\' => $tax_term_ids,
        )
      ),
    \'post_status\' => \'publish\',
    \'orderby\' => \'rand\',
    \'posts_per_page\' => 5
  );
  $my_query = null;
  $my_query = new WP_Query($args);
  if( $my_query->have_posts() ) {
    echo \'<ul id="related-posts" class="item-list">\';
    echo \'<h6>Related Posts</h6>\';
    while ($my_query->have_posts() && $counter < $post_count) : $my_query->the_post(); ?>
    <li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
    <?php
    $counter += 1;
    endwhile;
    echo \'</ul>\';
   }
   wp_reset_query();

}