Well, I don’t claim to be the great specialist about sound or zombies either, but here is my answer to your question part which is based on the logics of mine. It might have some holes, but I’ll try to concentrate on main points of the way I see this situation. This said, I have to say that I do not think those zombies have a chance of being concentrated at one same point unless being lead there manually or affected by some sort of magic/spell or collective consciousness. Here is why:
Firstly, if they get attracted by the sound, that means that any possible sound – i.e. wind playing with trees or plastic bag, rotten building falling apart, ocean waves or waterfalls, etc. – will catch their “attention”.
Secondly, you say: “Obviously they couldn't cross the ocean so is 6 continental clumps the best I could hope for?” – why not? Well, technically they will be carried away by the streams instead of “walking the ocean bottom” or swimming, but this will make all of them go to certain points where those streams pull stuff they carry. Unless sea life will eat them out before they reach the place, the streams carry everything to.
Also as far as I remember zombies do not make sounds themselves unless we talk about the sound of footsteps or bumping into something.
Finally, you said: “The only presumptions I want to make about the zombies is that they live forever aside from brain being damaged…”. 802
In your intro part you mentioned WD. In case it is the universe like WD then zombies do not live forever (unless magic, but that’s a different world). As the tissues of their bodies getting more and more rotten they will start falling apart and as their brain (or what’s left of it) is rotting as well – they will “die” eventually after a while (unless again – magic, which will turn them into ghosts, wraths and so on).
Plus the statement of “… they move toward the loudest sound they hear” also has some holes. Ok, they move. But simply moving towards something making a sound wouldn’t make zmbs any kind of a problem (unless traffic jams). What I mean is that they need some sort of a pointer to mark a target to attack. If the sound is the only marker, they are capable of recognizing (which makes it attack marker as well), those zmbs will eventually tear each other to pieces without any need for humans to interfere.
Thank you for reading and pardon for possible mistakes. All the best. 238