I've heard that roos cannot crow if placed in a coop that has a very low roof, apparently if they can't raise up their head they can't crow.
But, if you neighbor is not cooping them up, perhaps they are waiting for a pred to eat them...sticky wicket in any event....hope your talk works out with them...good luck.
I stayed with a friend in NC for a while learning the craft of broom making, they had Guinea fowl that roosted right outside my bedroom window, the first night I just about came unglued from the constant noise and finally got a long stick, opened up the window and chased them off every night just so I could get some sleep.
Don't know if even earplugs would work but you might try that. We had roos for one season or less who constantly crowed and drove me nuts, we gave them away and finally got some sleep. We now have only hens.