It's impossible to say what sound effect is played when you reach the -100 % mark, because the game is in Flash and the only thing you can do is to save the SWF file and then decompile it using an application that is developed for such processes.
I recommend Sothink SWF Decompiler since it gets the files exactly as they are, unless encryption has been used.
Download from here and load the SWF file into the program: http://www.sothink.com/product/flashdecompiler/
