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.