'þæt he fram Sigemundes secgan hyrde ellendædum' - 'heard tell about the brave deeds of Sigemund'.
The story of Sigemund is told in detail in the Old Norse Volsungasaga and the Middle High German Nieblungenleid. In Volsungasaga, Sigmundr (OE Sigemund) is the son of Volsungr. His sister, Signy, is forceably married to Siggeir. The father of Sigmundr and Signy, Volsungr, was treacherously slain at Siggeir's court with his other sons. Later, Signy disguises herself as a sorceress and seduces her brother, bearing him a son, Sinfjolti (OE Fitela). With Sigmundr, Sinfjolti slays Siggeir in vengeance. Father and son have many other adventures. Additionally, the scop here in Beowulf appears to attribute a dragon-slaying to Sigemund. In other accounts, another of his sons, Siegfried, kills this dragon.
For further discussion, see Ellis (1942), Garmonsway, Simpson and Davidson (1968), 252-64.