How do they find Axel when he is lost

They used voices to traverse the labyrinth, a technique call "acoustic arrangement".

"Keep up your courage," said my uncle. "As you are so weak, do not speak. We have been searching for you in all directions, both by going upwards and downwards in the gallery. My dear boy, I had begun to give over all hope—and you can never know what bitter tears of sorrow and regret I have shed. At last, supposing you to be still on the road beside the Hansbach, we again descended, firing off guns as signals. Now, however, that we have found you, and that our voices reach each other, it may be a long time before we actually meet. We are conversing by means of some extraordinary acoustic arrangement of the labyrinth. But do not despair, my dear boy. It is something gained even to hear each other."