This code isn't echoing out anything, even though I'm 100% positive $mixtape_id is set to an integer which corresponds to one of the posts within the 'mixtapes' custom post type
$query = new WP_Query(array(
'post_type' => 'mixtapes',
'p' => (int)$mixtape_id,
'posts_per_page' => -1,
'orderby' => 'meta_value_num',
'meta_key' => 'track_number'
));
while ( $query->have_posts() ) : $query->the_post(); ?>
<track>
<location>track1.mp3</location>
<creator>Artist name</creator>
<title>Track 1</title>
</track>
<?php endwhile; ?>
var_dump
?