function save_url_link( $post_id, $post ){
// only works if the post is an audio post format
if ( has_post_format('audio', $post_id)) {
// look for either http or https mp3, m4a, ogg, wav or wma files
$pattern = "/(http|https)://.*/(.*).(mp3|m4a|ogg|wav|wma)/";
// php the removes all except the mp3 link
preg_match_all($pattern,$post->post_content,$matches);
// saves the mp3 link to a string
$audio_raw = $matches[0][0];
// check if meta field is empty
if (get_post_meta( $post_id, 'audio_url', true ) == '') {
  // update the meta field
  update_post_meta( $post_id, 'audio_url', $audio_raw );
  // removes the save action
  remove_action('save_post', 'save_url_link', 10, 2);
  // update the content without the mp3 link
  wp_update_post(array('ID' => $post_id,'post_excerpt'=>$audio_raw,'post_content' => preg_replace('/(http|https)://.*/(.*).(mp3|m4a|ogg|wav|wma)/', "", $post->post_content)));
// close and save
};}}
add_action( 'save_post', 'save_url_link', 10, 2 );