http://www.zachpoff.com/software/video-trigger/
Attach a camera and trigger sounds (and more) whenever occupancy is detected in six user-defined areas of the video image. You have complete control over the playback logic and signal routing. Surround is supported. (If you want to trigger a video instead of sounds, see my other app,Video Triggers Video.)
I designed Video Trigger for artists who want to experiment with interactivity without building custom hardware or software. It aims to answer the question that I hear from many of my students:
“How do I make a sound play when somebody walks by?”
The answer often involves micro-controllers (likeArduino), or software (like Processing, pd, vvvv,Lily, Isadora, or MAX/MSP/Jitter). In the long-run these options provide much greater rewards, but if you have modest needs, Video Trigger is a painless way to get started.
It works with any Quicktime video input device, like the Apple iSight, DV camcorders, and industrial cameras. Video Trigger can play up to six sound files (AIFF or WAV, 1-16 channels) through any Core Audio sound hardware (1-16 channels) with complete control over signal routing.
NOTE: Video Trigger is not security software. It won’t email you when somebody breaks into your house. Try iSpy, BTV Pro, SecuritySpy, or Blue Iris. Also, MacTheClapper triggers an applescript whenever it hears a double-clap through your computer’s microphone! 2012 UPDATE: Video Trigger can now trigger an Apple Script or shell script, launch an application, open a file, etc. whenever a zone is entered or exited. See the guide for details.
VPT (Video Projection Tool) is a free app by HC Gilje that handles all kinds of complicated video projection tasks (cueing, mixing, effects, keystoning, etc…). He made a VPT extension based on Video Trigger so you can use a camera to trigger any VPT function.
Sometimes video motion detection is a complex and unreliable way to detect people. A simple floor switch or motion detector attached to a hacked computer keyboard or Makey Makey board could do the job nicely. Applications like the (free and wonderful) Soundplant will play sounds whenever a key is pressed, with fine control over the playback logic.