How to use EntityFieldQuery::fieldCondition for entity reference field?
You need to use 'target_id'

Your code should by this:

//query to get the list of entities
$query = new EntityFieldQuery();
$query-&gt;entityCondition('entity_type', 'song');

if(!is_null($form_state['input']['field_authors'][LANGUAGE_NONE]))
{
    $query-&gt;fieldCondition('field_authors', 'target_id', 
            $form_state['input']['field_authors'][LANGUAGE_NONE], 'in');
}


You can found a good explanation here:

How to use EntityFieldQuery with a simple text field?