Inadvertent horn :(

[firedoc]

I would not let them "play on" after the inadvertant horn. Some of the players would undoubtedly stop their activity giving the team with the ball an unfair advantage. Stop the play with a whistle and give the ball out of bounds at a spot nearest the ball's position to white since white had possession when the horn sounded. If the horn had sounded while the ball was loose, then the AP arrow would be the way to go.