why is it that the folks are usually holding a Playstation controller, yet the sound effects are ALWAYS from Pac-Man or the Atari 2600?

I've often wondered about this myself. This has really been going on since the days of the NES. Even if you can actually see the game that the characters are supposed to be playing, no matter game it is you hear Pac-Man. And not arcade Pac-Man, either. The unmistakable "dot dot dot" sound of 2600 Pac-Man.

This is my theory: The game's sound is probably turned all the way down while filming the scene, and then the "dot dot dot" sounds are added later by the sound effects guy. My guess is that the Pac-Man noises are already in the standard arsenal of effects (right between the gun shots and the Three-Stooges eye poke sounds), and that it is quicker and easier to use what they already have than to record the audio from the actual game.

Second theory, in case the first one proves wrong: if they used the audio and the video from the same game, they would violate the copyright of that game. But by mixing and matching two different games, they don't have to pay for the rights to either. This is much less likely to be the case than my first theory.