The biggest thing I added/upgraded to my haunt last year was sound! It's amazing what the hearing sensation can add to a persons mood, kind of like watching a movie before they add the soundtrack.
I'm not big on music in a haunt, unless it corresponds to something happening in the scene, ie; haunted piano/organ, radio on, etc. so I stick to the usual suspects; howling wind, thunder, screams, cats, phantom laughter, etc. A couple of years ago I tried this with an mp3 program that let you multi layer and add echo, pitch, etc. but it was only a 14 day trail, the cd did alright and I got some compliments but it didn't compare to last years sound track.
I just happened upon Audacity a completely free MP3 editor last year and used sounds that I found on the net. I couldn't believe this app. it was way better and had more features than the app. that wanted some $60 to purchase. It came out pretty nice and was very effective, I received tons of complements on the sounds. Also the free app. had size limitation, Audacity let me record the whole 3 hours (could have gone longer) as 1 track! This year I hope to record some of my own effects and completely redo the soundtrack yet again. The first year I used a cd but also last year I got to just use an MP3 player patched into a larger system that was hidden in a cabinet on my front porch.
Anyway I just thought I'd pass this along for anyone looking for a cheap way to add sound!
And you can check out some of my sound/effects track. in my video on youtube: Zombiehorror's Halloween Graveyard 08
Hope this info helps some fellow haunters this year!