// Filter wp_nav_menu() to add an additional span element
function span_nav_menu_items($items) {
	return preg_replace('@\<li([^>]*)>\<a([^>]*)>(.*?)\<\/a>@i', '<li$1><a$2><span>$3</span></a>',$items);;
}
add_filter( 'wp_nav_menu_items', 'span_nav_menu_items' );