You probably have a copy of Incoming, and have played it for a grand total of 1 hour. Because after you get used to the nifty effects, shooting down wave after wave of bad guy gets boring. More on Incoming later. That said, graphics can do a great deal to increase the fun factor of a good game. I don't think people would buy 1 million copies of that.
Pretty graphics are an enhancement.
Sound is a similar beast. Without sound, a game isn't going to be very popular. 3D positional sound is a buzzword right now, and while it's cool and does adds to immersion, it isn't as vital as you might have been led to believe. Gameplay is a number of things, the way the controls work with your character onscreen, the way your character interacts with the environment, and the way this interaction makes the living breathing person behind the monitor feel. These feelings can range anywhere from pleasure or satisfaction to frustration and anger.