Can’t save php string to a custom field

For some reason, your $subject variable is empty when you call get_the_content($post_id).

When using save_post hook, you can pass additional arguments like $post and $update to your callback.

For your case, you could use the second argument $post to grab the post contents.

Code:

function save_url_link( $post_id, $post ){
    $pattern = "/(http|https):\/\/.*\/(.*)\.(mp3|m4a|ogg|wav|wma)/";
    $subject = $post->post_content;
    preg_match_all ( $pattern, $subject, $matches );
    update_post_meta( $post_id, 'audio_url', $matches[0][0] );
}
add_action( 'save_post', 'save_url_link', 10, 2 );

The above code uses the logic you implemented, with some differences.

1. Now we are using the second parameter $post in the callback, and we’re picking up the submitted post content via $post->post_content.

2. add_action‘s fourth argument is the the number of arguments your callback will take. Default is 1. We are using 2 because we now have $post_id and $post.