if($_product->getTypeId() == 'downloadable') {
    /** @var Mage_Downloadable_Model_Resource_Sample_Collection $samples */
    $samples = Mage::getModel('downloadable/sample')->getCollection()->addProductToFilter($product_id);
    if ($samples) {
        $_myprodsamples = Mage::getModel('downloadable/link');
        $_mySampleCollection = $_myprodsamples->getCollection()->addProductToFilter($_product->getId());
        foreach ($_mySampleCollection as $_sample){
            $_samplelink = $this->getUrl('downloadable/download/linkSample/link_id/'.$_sample ->getId());
        }
    }
}


<?php if($_samplelink): ?>
  <a href="<?php echo $_samplelink ?>" class="button btn-cart piwik_download" target="_blank">Downnload</a>
<?php else: ?>