Custom field picture instead of post thumbnail on category page

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.