Well, I would like to add some input on the visuals. One thing to do is get the players attention, like a row of boxes, that slowly goes bump, like something is behind them or have something fall over. Have simi lit areas the lighting changes or dim alternating lights that fade in and out, then have a moster in show up in area that was once dark and now lit. Things like windows that break with limbs or boards that fall for no reason can be a tension builder. I always like the dead bodies in hard to see areas, you can never tell quite what happened, so the player is not sure what is in store for them. Have doors or path get blocked so the player is force to move into a scary area. Having the player in peril is good too, like slowly moving crush walls where the player is forced to think quickly to esape or push the player into a mob of zombies. Water can be very scare too, you never know what is underneath. Create disorder by having objects fly around like boards or garbage, so the player cannot focus in one area or on the monster. Another thing I like is something is like a car lot where you can just see mob of moster coming around the cars, give the player room to move, but have the mosters come from everywhere, encircling the player. A couple of other tricks would to force the player backward to a ledge that breaks off because the mob of mosters is to overwhelming or jumping to a cliff that breaks offs and sends the player plunging. And last but not least anything where the player has little or no control.
Damn, now I want to make a scary SP level. Hope you get a couple of ideas from this.
"When one person suffers from a delusion it is called insanity. When many people suffer from a delusion it is called religion." -Robert Pirsig