In Vienna 1975, why does Paul find it impossible to enjoy the music he hears in Salsburg?

I believe Paul finds it impossible to enjoy the music because it is in Salsburg that he finds out the truth about Keller's past. Although he'd learned much about the city and the music from his teacher, he'd learned nothing about the pain and suffering his mentor had experienced there. The realization was bitter sweet and supplied him with a completely different perspective.

From Gradesaver's summary;

According to Henisch, when the Nazis arrived, people began to leave and to disappear. Henisch said he stayed because he did not have enough money to leave. Paul asks if Keller tried to hide his wife, but Henisch says no. Adolf Eichmann heard Keller play in Vienna in 1938, and Keller was flown to Berlin several times to play for him and Adolf Hitler. He attempted to make himself so visible that no one could touch him or his family. However, his plan backfired and his wife was captured while Keller was in Berlin for his last performance for Hitler. Eric refused to be separated from his mother and was captured as well.

When Keller returned home, he sewed a yellow star to his clothing and registered as a Jew. Keller was sent to the Auschwitz concentration camp in 1942. Henisch says Keller was part of the prisoners forced to march from Auschwitz to Buchenwald to Bergen-Belsen, and that he died on the last journey. Paul asks how he knows this, and Henisch unbuttons his sleeve to show his own six-digit tattoo.