Wordpress wrap all vowels in the_title within <span> tags

function str_replace_assoc(array $replace, $subject) {
   return str_ireplace(array_keys($replace), array_values($replace), $subject);
};

function title_wrap_vowels($items) {

$vowelreplace = array(
'a' => '<span>a</span>',
'e' => '<span>e</span>',
'i' => '<span>i</span>',
'o' => '<span>o</span>',
'u' => '<span>u</span>',
);

return str_replace_assoc($vowelreplace, $items);
};

function title_wrap_vowels_menu($item_output, $item, $depth, $args) {

global $wp_query;
$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

$class_names = $value = '';

$classes = empty( $item->classes ) ? array() : (array) $item->classes;
$classes[] = 'menu-item-' . $item->ID;

$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
$class_names = ' class="' . esc_attr( $class_names ) . '"';

$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

$output .= $indent . '<li' . $id . $value . $class_names .'>';

$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
$attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

$item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        @h@$item_output .= $args->link_before . title_wrap_vowels(apply_filters( 'the_title', $item->title, $item->ID )) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;
    return $item_output;

}
add_filter('walker_nav_menu_start_el','title_wrap_vowels_menu', 1, 4);