Multiplayer is rated based upon the number of players a game supports simultanously, any changes in the gameplay from the single-player experience, game balance, setup and fun factor. Titles which do not have a multiplayer component are not rated on this criterion. Sound FX refers to the digitized sounds you hear during the game such as speech, weapons fire, etc. Also, points are given for sound depth, timing, quality, and appropriateness. Also taken into account is whether the score adds or detracts from the game.
Difficulty refers to how, overall, challenging a game is.
However, if you are slaughtered consistently, that title is rather difficult. This is more a measure for advanced, hardcore gamers who like a significant challenge. This criterion is meant to state whether the game was poor, average, good, etc. That's what the Overall score is meant to help answer.