Theon would have met Stannis before this scene when he was brought before him. I don´t think that this scene is the first time when Theon is awake in Stannis camp. Crowfood Umber would have brought Theon and "Arya" before Stannis. Being chained up in Stannis´solar, Theon would´ve witnessed quite a lot of Stannis´actions so he would know Stannis voice when he hears it. I think Tybald was indeed the maester of the Dreadfort. Roose wanted his maester as a spy inside Stannis´ army, so he sent him to Arnolf Karstark so that Tybald could take the alias of the Karhold´s maester.
