Problem in displaying related posts by specific taxonomy in custom post type

I’m trying to display related posts by (specific) ‘post_tag’ taxonomy in custom post type. Following code is displaying all post by a custom post type.

I mean i want to display related posts by post tag of ‘Mp3‘ custom post type in its single post page.

$post_type="mp3";
    $tax = 'post_tag';
    $post_count = 5;
    $counter = 0;
    $tax_terms = get_terms($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',
            'showposts' => 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="https://wordpress.stackexchange.com/questions/237220/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
            <?php
            $counter += 1;
            endwhile;
            echo '</ul>';
         }
         wp_reset_query();
        }
    }

So I’m here asking for solution…

Try this:

On the single view, you need to use get_the_terms() rather than get_terms(). That way, you’re getting the terms that are actually assigned to the post, rather than all the terms that exist.

    $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="https://wordpress.stackexchange.com/questions/237220/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
        <?php
        $counter += 1;
        endwhile;
        echo '</ul>';
       }
       wp_reset_query();
      }
    }

NOTE: This code will produce multiple lists of related posts. If the current post has 2 tags assigned to it, you’ll get 2 lists of related posts. If you’d rather nave one list of related posts from both tags combined, you’d use something like this:

$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="https://wordpress.stackexchange.com/questions/237220/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
    <?php
    $counter += 1;
    endwhile;
    echo '</ul>';
   }
   wp_reset_query();

}