Snakes have no visible ear, so they don't hear sounds as people do.

They have vestiges of the apparatus for hearing inside their heads, and that setup is attached to their jawbones, so they feel vibrations very well and may hear low-frequency airborne sounds.

It's thought that the hissing sound snakes make is purely to warn other creatures because presumably, snakes can't use sound to communicate with each other.

More Info: www.livescience.com