Kristoff supplemented his singing with music from his lute.
One of Kristoff's possessions was a lute.
History
As Kristoff and Sven settled down for the night in a barn next to Wandering Oaken's Trading Post and Sauna, Kristoff sung a lullaby using his lute as an accompaniment.
While traveling to the North Mountain, Anna used the lute as a makeshift weapon to defend Kristoff's sled from an attacking wolf. The lute was ultimately lost when Anna, Sven, and Kristoff were forced to abandon the sled upon encountering a gorge, though the princess promised to replace everything lost. True to her word, Anna gave Kristoff a new lute along with his new sled.