Bad ending
There is no way around it, the ending sucks.
As another commenter pointed out, the guns would have to be reloaded with fresh shells several times. And even a giant house could not muffle a shotgun blast. The fact they barely heard it is just ridiculous. I live in an area with a lot of hunting, and I can hear shotguns from at least half a mile away.
Also, about the boombox left recording... Some of us poor people used to do that in the Nineties to get free music saved on cassette, but they were talking over it so it was useless anyway, and well, they are incredibly rich. The film makes a point that they have CDs and really nice electronics, so why would she record a crummy version off of the TV? I think they even mention specifically that they have the CD to that very same opera they are listening to. Why is she recording it!? It makes absolutely no sense. It's these obvious gaps in logic that annoy me in movies.