on the webassist/plugins/shopping_cart/add_to_cart.php page, you can edit the sold out message by editing the following line:
<div id="soldOut" class="alertText" <?php echo(($row_NumLeftRS["NumLeft"] == 0 && $row_WADAProducts["ProductLive"] != 0 && $row_WADAProducts["ProductUnlimited"] == 0)?'':'style="display:none"'); ?>> Sold out! </div>