I remember watching a making of dvd of some PS2 game I bought back in 2004 (think it was called the sufffering?) and it showed how they made the gore sounds. It showed a dude with a sledgehammer breaking all sorts of fruit, meat, and other things to make those extra icky squishy sounds.
I wonder what they did to make the heads go splat sounds. My heart of hearts wishes its them doing something completely ridiculous and off kilter, but my dreams have a way of being destroyed.