Snakes can "hear".. But they don't listen for crap..
Seriously though, snakes can "hear" sounds in the range of low hundred hertz.. A human voice can range anywhere from 100hz to 10Khz (Mariah Carey or something). The lowest audible sound that can be heard by humans is 20hz.. So scratching your head can be at least in the hundreds.
Like snakehunter stated, the lower jaw is sort of like an antenna which receives the low freq. waves and translates that into what the snake "hears".
I don't think it is but I apologize if any of my input is wrong.:w