Play the movie. During the party, play a DVD of various horror movies. You don't need to turn up the volume to get the effect. If you play the movie that inspired you to decorate the space, people will more easily recognize the scenes you have recreated.
Use a soundtrack. Download scary music from iTunes. You can also play the soundtrack and sound effects from famous movies. The music in Jaws as the shark approaches is a classic. Another horror movie staple is the high-pitched violin heard in the movie Psycho.