There are several ways to go, depending on your level of technical knowledge. Last year, around Christmas, I saw several "effects kits" for sale at chain stores like Wal-Mart, Target, Fred Meyer, etc. These are simple kits that can set up audio and lights to run in a sequence. They're mainly designed for the musical christmas "dancing lights" kinda stuff, but you could set the queues to halloween music or noises.
Another route to take is setting up individual props with motion detectors. This is pretty easy, and motion detectors are now really cheap. You just set up a prop with a sound or light effect, and the motion sensor completes the circuit.
If you want to get more complex, you can bring in a computer. There are many programs that can run multiple audio/visual queues. You may also be able to get your hands on an effects board (check eBay or CraigsList) which would allow you to wire multiple events and control them from a single station or program in a looping series.
If you run a search on google using key words like "yard haunt", "effects board", "theatre effecs"
Also, check out these sites:http://www.hauntershangout.com/props/default.asphttp://www.yardhaunter.com/http://www.hauntedhousesupplies.com/http://www.scary-terry.com/