Change the default value of a checkbox element
This code find the checkboxes in the 'submitted' element and check it by default.
If you make a screenshot of dsm($element['submitted']) message I can check if this code is correct ^^ 

function module_form_webform_client_form_31_after_build($element, &amp;$form_state) {
  foreach ($element['submitted'] as $key =&gt; &amp;$value) {
    if ( $value['#type'] == 'checkbox' ) {
      $element['submitted'][$key][1]['#checked'] = TRUE;
    }
  }
  return $element;
}