He's a great musician, the interpretation of the viking age on his music is really awesome, but how historically accurate is it? Is there any record of how music was in the viking age (and before) and in that case, how much does Einar draws from it?
Short answer: It's not.
Short answer, but longer: We have no extant musical notation from viking-age Scandinavia which he can draw on to create his songs. His music is based on, essentially, vibes and modern interpretations of the viking era. It's cool, but it's not what Ingólfr Arnarson would have been listening to when he settled Iceland.