Adding Span element within anchor element in a custom menu

I have created a custom menu with machine name menu-cmenu and printing it in page.tpl.php with:

$cMenu = menu_navigation_links('menu-cmenu');
print theme('links__menu_cmenu', array('links' => $cMenu));

After reading several threads similar to this, I have created a function named zenc_menu_link in template.php which is supposed to add span tag. But after debugging the code, I find out that zenc_menu_link is never actually called for my custom menu menu-cmenu.

I need some suggestions please to rectify the issue.