How do I construct a mirror with a sensor and speakers?

The standard laser detector (like the one used on urinals) would suffice, but instead of triggering a flush, trigger a pre-recorded voice. You may be able to find something here that would work.

Just scrap out one of those halloween things tha triggers when someone walks by with an ultrasonic sensor, and take the leads from the lights to activate a relay that hits the play button on some sort of recorder.

Something like this?

@digitalimpression: I think those detectors are PIR – passive infrared, which pick up on body heat by sensing infrared light emitted by the subject, but they don’t contain lasers. Same as motion detectors in security systems.

Using digitally stored sounds requires only an electrical trigger to re-play from a device like in @digitalimpression‘s link. The trigger can be generated by a PIR motion detector such as this part (click on “data sheet” for more info).

At first I was thinking an Arduino board would be an excellent way to go, though I’ve never actually worked with one and I’m not sure I would know how, but this guy has another way that you might consider. It does use a PIR sensor but I’m not entirely sure what the problem with that would be anyway.

I would strongly recommend searching the Instructables site that @digitalimpression linked to. Lots of amazing ideas there, and this one I’m posting just because it’s clever, I think, where someone uses the motion detecting system in an Airwick Freshmatic Compact i-Motion air freshener. It might be useful, just a way of thinking of how you might do this project reusing available hardware if you are handy with electronics, and I think you would have to be to do this yourself.