. >> reporter: whenever he can, murry sidlin urges them to do more, because what they're rehearsing, what they are trying to commemorate, is another performance by another choir in horrific circumstances: jewish prisoners in a nazi concentration camp. >> to us it's just damn words. they leave the rehearsal and walk over bodies to get back to their barracks. we cannot be indifferent. >> reporter: this music, verdi's lyrical mass for the dead, is a full-throated testament to the majesty and judgment of god, profound even in this rehearsal at washington's kennedy center. but it was perhaps never more powerful or poignant than its performance on june 23, 1944 in the concentration camp, terezin, just outside prague. when jewish prisoners sang the requiem to their nazi captors, that catholic mass, says terezin survivor vera schiff, gave prisoners a way to defy the nazis. >> the text of the latin prayers suggests that we all will be judged by the almighty, and this would include the germans. that was a promise. that the day will come when we will all be facing the ultimate, the final judge,