Potresti lanciare la funzione sum() in questa maniera:
codice:
<script type="text/javascript">
$(function(){    
 $('.factor').keyup(function(){   
  var t=$(this).parents('tr')        
  var factors=$('.factor',t);    
  $('input[name^="imp_"]',t).val(Number(factors.eq(0).val())*Number(factors.eq(1).val()));
  
  sum();

 });
});
</script>
Oppure, calcolando in maniera meno verbosa:
codice:
<script type="text/javascript">
$(function(){    
 $('.factor').keyup(function(){   
  var t=$(this).parents('tr')        
  var factors=$('.factor',t);    
  $('input[name^="imp_"]',t).val(Number(factors.eq(0).val())*Number(factors.eq(1).val()));
  var total=0;
  $('input[name^="imp_"]').each(function(){
   total+=Number($(this).val());
  });
  $('input[name="totale"]').val(total);
 });
});
</script>