Sound FX refers to the digitized sounds you hear during the game such as speech, weapons fire, etc. The more creative, realistic, and distinct the sound, the higher the score. Use of 3D sound such as A3D, EAX, DirectSound 3D are also noted. 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 game 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 offering was poor, average, good, etc. That's what the Overall score is meant to help answer.