If you want to make the list of signatures displayed on your site sort by last name;

In the file /wp-content/plugins/speakout/includes/class.signature.php find at about line 76

ORDER BY $db_signatures.id DESC $sql_limit

and change it to

ORDER BY $db_signatures.last_name ASC $sql_limit

to order by last name ascending. If you want descending

ORDER BY $db_signatures.last_name DESC $sql_limit

This file is updated pretty rarely, if ever, so your changes should be pretty sticky.