Hmmm.... sounds like maybe your tpl_modues_youtube.php is in the wrong place:
../includes/templates/your_theme/templates/tpl_modues_youtube.php
is working for me, both online and at my local server. Have you edited your tpl_product_inf_display.php file to include the line
<?php require($template->get_template_dir('/tpl_modules_youtube.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_youtube.php'); ?>
AND placed it in includes/templates/your_template/templates/ ?
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