Have you tried it using a WP_Query? Like this for instance:
<?php
$speakers = get_field('speakers_two');
$query = new WP_Query(array(
'post_type' => 'speaker_info',
'post__in' => $speakers,
'meta_key' => 'last_name',
'orderby' => 'meta_value',
'order' => 'ASC',
));
if ($query->have_posts()) : ?>
<ul>
<?php while($query->have_posts()) :
$query->the_post();
?>
<li>
<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
<span>Custom field: <?php the_field('last_name'); ?></span>
</li>
<?php endwhile; ?>
</ul>
<?php endif; ?>
Let me know if it works or not, it did the trick for me with a relationship field I wanted to sort.
