They can be ear piercing.Loud enough to scare me to death if I'm not expecting it!
But honestly, my Cayuga ducks quack more often than the calls. Mine are pretty quiet the majority of the time. They quack when they see me or need food. They quack if something is going on..like they see a turtle or something. But they don't normally just quack to be quacking. I think a couple of my Cayugas just like to hear themselves.