Motion Sensor Mp3 Stereo PLayer

How would I would I go about making a Motion Sensor Mp3 player using my Stereo.  I want to have the ability to play an mp3 file using my Stereo when people pass nearby the sensor.  I want something loud and very realistic.  I am unsatisfied with the reviews of the below mentioned products.  
I want to be able to play the mp3 file - motion sensor using my stereo player.

I'm in the process of making a motion sensor audio/video player using a PIR sensor and the Raspberry Pi. The Pi is then hooked up to an amp via the 3.5mm jack to stereo RCA. Whether this is practical for you depends on what you're intended implementation is, I'll be posting the Instructable within the next couple of months.

Hopefully someone else will have some other ideas too.