Alexander Rybak performed at the Norwegian game exhibition Spillexpo 2014 . He played the Super Mario Theme song on violin, and you can watch the performance and the interview afterwards with VGTV in the videos below.
Found by Mari Hanssen, translated by TessaLa, revised by Katie Anderson
Spillexpo said Alexander’s performance of The Super Mario Theme song would be uploaded on YT. Not sure if this is the version they talked about. But anyway, thanks to Scaryplayer for this video.