You determined when the $show_mp3player is true or false ... but you did not apply the results to running or not running the sidebox ...

Code:
if ($show_mp3player == true) {

  require($template->get_template_dir('tpl_temps.php',DIR_WS_TEMPLATE, $current_page_base,'sideboxes'). '/tpl_mp3_player.php');

  $title =  BOX_HEADING_MP3PLAYER;
  $left_corner = false;
  $right_corner = false;
  $right_arrow = false;
  $title_link = false;

  require($template->get_template_dir($column_box_default, DIR_WS_TEMPLATE, $current_page_base,'common') . '/' . $column_box_default);
}