Another great resouce for Editing, Creating, Modifying sound files is www.goldwave.com this is an excelant tools for all your sound needs.  

Hi Oy, the way that I've done this is to take teh sound file and make several copies of it from GoldWave and increase the amplitude a little each time I copyit into seperate files like Sound1.wav,Sound1.wav,Sound2.wav,Sound3.wav,Sound4.wav,Sound5.wav and call them in sequence as the object get closer: you can get goldwave at: ...