In the Volsunga Saga, there are two Sigmunds. The first is the son of Volsung and becomes king of Hunland. Sigmund Volsungsson also is the one the that draws the sword from the Branstock. Siggeir places Sigmund Volsungsson in the stocks where he escapes before being killed by a she-wolf. There are other adventures described in the Saga, including an episode where he tests and kills Signy's sons by Siggeir. Sigmund Volsungsson mates with Signy and procudces one son.
Sigmund Sigurdarson is the son of Sigurd and Gudrun and Brynhild kills him. Sigmund is only three when he is killed and placed on his father's burial pyre.
Information for this node taken from The Volsunga Saga, by Penguin Classics.