Our haunt is outdoors and wraps around the entire house (half acre lot). The approach is up the driveway, so we have a stereo system pumping out spooky Halloween music to set the mood.
Throughout the rest of haunt we have several small systems built from powered computer speakers and $10 mp3 players. The computer speakers were purchased from a used computer store for $5-10 a set. I cut the wallwart away from the power plug and replaced it with a 9 vdc battery clip which allows them to be placed just about anywhere. Painting the speakers black makes them pretty easy to hide behind tombstones, under bushes, or behind/up in trees. These small systems play sound tracks of random noises such as rattling chains, dogs barking, distant thunder, screams, boinging springs, loon calls, laughs, giggles, and such. The sounds are usually spaced about 5-10 seconds apart and alternate between the left and right channels.
We tune/tweak each system's sound track to match the area of the yard it's in. For example, the graveyard is biased towards screams, moans, rattling chains, digging sounds, and such while around the pond different splashing sounds are mixed in.
Works pretty good. We get a lot of good comments about the sounds.
Chivalry is NOT dead...but I'm working on it.