I reckon you use the image URL in your custom field.
$metro_creativex_posttitle = get_the_title(); // Not used in this snippet
/*
// you don't need this block
$metro_creativex_feat_image = wp_get_attachment_image_src(
$post->ID,
get_field('sd_single_img'),
'single-post-thumbnail'
);
*/
// you'll need `$post_id` for each song
$img_src = get_field('sd_single_img', $post_id);
if( !empty( $img_src ) ) {
// echo if custom field is not empty
echo '<div class="img"><img src="https://thenewblackgold.netsons.org/wp/category/songs/level-5/' . $img_src . '" /></div>';
}
Updated in compliance of the OP comment.