You are correct that it was Juliek, a tiny young Polish boy--who recognized that he would not survive the death march--begins to play Beethoven on the violin. It is a marked contrast to the situation they are in, and to the main character/author; this makes it all the more beautiful.
It is a very short section of the book, which is cut off abruptly when Elie awakens to find young Juliek dead, his violin smashed to pieces. It is perfect symbolism of the hopelessness of their situation, and Juliek had accepted this hopelessness, deciding to exit the world in the most beautiful way that he could.
He played violin