jquery slide toggle - affects all on the page
You'll need to make use of the this keyword inside your click function:

 Drupal.behaviors.myToggle = {
   attach: function(context, settings) {
     $('.views-row').find('.views-field-field-mp3').hide();

     $('.mp3-toggle', context).click(function() {
       $(this).parents('.views-row').find('.views-field-field-mp3').slideToggle();
     });
   }
 };