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If you go to radio shack they have a voice chip on clearance for $3, down from $20. It holds 20 sec. of whatever you want and plays it back whenever triggered. It's just the chip itself, but you said you had experience with electronics so it shouldn't be too hard.
It'd be pretty involved to make it wireless, so I'd just go with a signal wire. As far as the sensor goes you can build one with IR diodes that can detect movement in front of them. There are plenty of simple schematics out on the net that can instruct you in their construction.
I'm out of town at the moment, but I can try and post something when I get back to my apartment.
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"Empirically observed covariation is a necessary but not sufficient condition for causality" - Edward Tufte
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