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);
}