What image does Eliezer see whenever he hears a particular piece of music by Beethoven?

Elie will always envision Juliek when he hears that particular piece of music by Beethoven.

I shall never forget Juliek. How could I forget this concert given before an audience of the dead and dying? Even today, when I hear that particular piece by Beethoven, my eyes close and out of the darkness emerges the pale and melancholy face of my Polish comrade bidding farewell to an audience of dying men.