"Con los delfines" and "Terapia de soledad" are both from the Buzon de tiempo section of Buzon de tiempo. This section is made up of letters (or their equivalent).
"Con los delfines" returns to the Dirty War theme of "El diecinueve." It is a letter written by a young woman who was raised by a couple who were not her parents. She learns that she is actually adopted and that her parents were disappeared from their apartment and later dropped into the Rio de Plata. (I don't know what the estimates are, but the situation of infants being taken from parents who were disappeared was a real one.) The letter is being written to her adoptive father, who obviously knew where the infant he was adopting had come from, telling him how she found out and how consequently she does not ever want to see him again.
"Terapia de soledad" is a letter written by a woman coming home to her husband after having spent some time in the woods away from everybody. It's a pean of sorts to the joys of solitude. The theme of solitude is one of the themes that reappears frequently throughout the collection. Here it is fairly positive, a sort of chance for spiritual renewal.