I used this sharp distance sensor http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp2y0a21yk_e.pdf
(Adafruit) to do something similar. I was a little nerdy about it: I used the Vout vs Dist chart in the datasheet, plugged the x/y values in a graphing calculator and determined the best fit line, then coded the output voltage into the "best fit" equation to find the distance.
An easier way would be to measure a distance, say 5 meters in .5 meter increments and measure the output at .5m, 1m, 1.5m, 2m, etc.
What mp3 player did you end up buying? If it's the mp3 Trigger (Spark Fun) the set up is pretty easy. Connect your Vcc, grn, and the RX and TX to Arduino and use serial commands to the trigger. Their datasheet explains all the commands.