Dear Diary:Soon after we moved into the older apartment in Yorkville with walls that turned out to be thin, our Saturday afternoons were interrupted by opera music coming from our neighbor's apartment.I knew right away that he was listening to "Live at the Met" broadcasts. Fortunately, because I enjoy opera, I did not mind the intrusion. But I could forget about getting any work done at home.Only occasionally did we catch a glimpse of the neighbor in question. He was short, with white hair and a goatee, and he rarely left his apartment. When he did, he was dressed in a rumpled suit and tie, with an overcoat and a beret and a wire shopping cart trailing behind.When we exchanged a few words of greeting, I detected an accent that sounded as though it might be Eastern European. I decided he was probably a holdover from the days when the neighborhood had a significant German and Hungarian population.One Saturday afternoon, there was complete silence. Concerned, I found the superintendent, who was no doubt familiar with our neighbor's routine. With some trepidation, he opened the door, and we entered the darkened apartment to discover the man's apparently lifeless body sprawled near his radio.I called 911. The police and firefighters arrived, and I was questioned by a veteran officer who asked me how I knew something might be wrong."I didn't hear the music," I stammered.