<li class="dropdown">
  <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Produtos<span class="caret"></span></a>
  <ul class="dropdown-menu">

   <li class="dropdown dropdown-submenu"><a class="dropdown-toggle" href="#" data-toggle="dropdown">Audiologia</a>
    <ul class="dropdown-menu">

      <?php

      require 'conexao.php';

      $consulta = $PDO->query("SELECT * FROM subcategoria WHERE categoria_id = '1' ORDER BY ID;");

      while ($linha = $consulta->fetch(PDO::FETCH_ASSOC)) { ?>

      <li><a href="produto.php?id=<?php echo "$linha[id]"; ?>"><?php echo "$linha[titulo]"; ?></a></li>

      <?php } ?>

      <ul class="dropdown-menu">

        <?php

        require 'conexao.php';

        $consulta = $PDO->query("SELECT * FROM produto WHERE categoria_id = '1' AND subcategoria_id = '1' ORDER BY ID;");

        while ($linha = $consulta->fetch(PDO::FETCH_ASSOC)) { ?>

        <li><a href="produto.php?id=<?php echo "$linha[id]"; ?>"><?php echo "$linha[titulo]"; ?></a></li>

        <?php } ?>

      </ul>

    </ul>
  </li>