your sort link need to include the category
your sort is working by passing the sort URL variable:
sort=2015_Awards_Participant
where you are passing in the category as another URL variable:
CategoryID=7
you need to concatenate the 2:
CategoryID=7&sort=2015_Awards_Participant
so fior example, the link would need to look like this:
<a href="index.php?CategoryID=<?php echo(isset($_GET['CategoryID'])?$_GET['CategoryID']:""); ?>&sort=2015_Awards_Participant">