Using computed fields module to display certain fields?
Sorry for the late response, but this is an issue I struggled with as well.

In the first box, 'Computed Code' you determine the value of the computed field based on what is present in the $entity object. Calculations are possible, but drupal functions or methods are not available. In your case the entire determination can be performed here:

if ( isset($entity-&gt;field_shortdesc) &amp;&amp; trim($entity-&gt;field_shortdesc[$entity-&gt;language][0]['value']) != "") {
  $entity_field[0]['value'] = $entity-&gt;field_shortdesc[$entity-&gt;language][0]['value'];
} elseif (isset($entity-&gt;field_audio_description)) {
  $entity_field[0]['value'] = :$entity-&gt;field_audio_description[$entity-&gt;language][0]['value'];
} else {
  $entity_field[0]['value'] = "";
}


The second box, 'Display Code', is for processing the value you determined in the first box. If you have to call drupal functions to load a user object or to format text this is where you would do it. In your case, no further processing is required, so you would leave the default:

$display_output = $entity_field_item['value'];


Then if your fields are using an input format (eg. 'Filtered HTML') you will want to set the display format (under 'Manage Display') to 'Filtered markup'.