Rewriting the WP Sub Posts plugin to allow one custom post type to be a child of another custom post type

Haven’t tried this, but if you look at the code, there’s a query where the posts are retrieved from the database:

$query = "SELECT * FROM $wpdb->posts $join WHERE (post_type="post" AND post_status="publish") $where ";

As you can see, it’s looking for post_type="post". So this will only get the default ‘post’ type posts. You can modify it yourself to put ‘track’ or ‘mixtape’ there, but I would also recommend contacting the original developer and ask him/her to make the post_type a parameter in the plugin.

Hope this helps!