How can a severed head still make the gurgling, throaty growls and grunts without any lungs? For that matter, how do skeletal zombies make the noises as well?
There is no canonical - or logical - explanation for this. On a minor note, the producers have said that they will never show us a zombie so decayed as to be purely skeletal - they will always have enough muscle tissue to move around, if they are moving around.
But when we see a zombie head, and hear it growling or moaning (season 4, episode 1, the Irish girl's camp comes to mind), it is merely for effect, and doesn't make any sense, nor is it probably supposed to.
Fortunately for everyone involved, this rarely comes up.