"Snakes are deaf to airborne sounds. Thus a rattlesnake does not hear the sound of another snake’s rattle, nor does the cobra hear, the snake charmer’s flute. They can, however, feel vibrations through the ground or whatever they are resting on. "
taken from this web page
http://www.manbir-online.com/snakes/snake.55.htm

In short,if your throat is close to the snake they are able to feel the vibrations from your vocal chords, or if you are playing your radio loudly they are able to feel the vibrations from the bass, which can cause the snake to stress out/act crazy.