It is a door chime, and it is obnoxious. I'm not sure which video you watched, but I tried to mute it in mine, but couldn't get satisfactory results.
There are currently four books he wants to write after he finishes Leviathan. He is not prepared to talk about them yet.
He knows there's interest in another Trevor Lawson, and a couple of months ago, he told me he'd been thinking about that again, and maybe after he finishes Leviathan....
So basically, we'll see what happens.