Sjøberg was a shoemaker for seven years before studying voice on a scholarship at the Royal Opera School in Copenhagen,
with Vilhelm Herold. He made his debut as a baritone in May 1937 in Copenhagen (as Silvio
in Pagliacci); for the next six years, he sang mostly small baritone parts.
In 1943, he went to Stockholm to restudy as a tenor with Joseph Hislop, and
made his second (tenor) debut in December 1944 as Don José, again at the Royal Danish Opera Copenhagen, where he spent
his entire career. His repertory included David (Saul og David by Carl Nielsen), Pedro, Canio, Peter Grimes (perhaps his
most famous role), Fenton (Falstaff), Radames, Bacchus, Lohengrin and Stolzing. He made a few guest appearances mostly as
Lohengrin, in Lyon, Toulouse, Rome and Stockholm. He retired in 1959.
Reference 1 (and source for the recording and pictures);
reference 2: Kutsch & Riemens