She let out a scream of mirth that caused Hedwig to wake up and flap her wings indignantly and Crookshanks to leap up into the luggage rack, hissing.
Harry had seen enough; laughing, he ducked down low, ran to a door he knew was concealed behind a tapestry a little way along the corridor and slipped through it to find Fred and George hiding just behind it, listening to Umbridge and Filch’s yells and quaking with suppressed mirth.
There are no more uses of "mirth" in the book.
Show samples from other sources
It was an evening of many stories and great mirth.
He felt no mirth or humor and wondered if there’d ever be a time again when he would.