You can do this by going to
\The Game Creators\FPS Creator Reloaded\Files\gamecore\guns\modern\Shotgun
Just replace the .wav with the one you have make sure the name is the same and around the same size.
(And always backup)
PC SPECS: Windows 8 Pro 64-bit, Intel I7 4GHz CPU, Asus R9 3GB GPU, 32GB DDR3 MEM