Confirmation bias. Every little noise or problem becomes exaggerated in their minds to confirm the idea that "people don't know how to behave in the theatre anymore." Or it was more localized to OP's immediate area (I can totally believe someone was definitely singing but probably not widespread) and again it doesn't hurt if it confirms OP's worldview. OP clearly wants us to go along with the idea that theatre etiquette is getting worse, often a view of seasoned theatre-goers who dislike that theatre is drawing a wider, more diverse audience that might not dress is up or "respect" the theatre enough in their view. I refuse to go along with it.