Nick is going around with a shopping cart collecting Walker heads so he can sell them for brain stem harvesting. (Who, BTW, was the first person to decide it was safe to try eating brain stems of dead Walkers? And never mind that human brain stems aren't even a source of adrenaline.) The severed heads continue to make the typical Walker growling noises. Which would be impossible without a source of air passing over the larynx. The only sound a severed head could make would be a gnashing of teeth.