An inadvertant horn is addressed in 2011-12 Casebook 2.11.3, and refers to the horn sounding during a live ball:
"If the scorer signals while the ball is live, the official should ignore the signal if a scoring play is in progress. Otherwise, the official may stop play to determine the reason for the scorer's signal."
I can't find your exact situation, but the officials must have felt the horn happened before, or was close enough to being prior to, the release of the free throw that they stopped play. It would be interesting to see if they would have counted a made 1st free throw. I'd be inclined to think they would have. As I said in my first post, the officials could have felt the horn effected her missed shot. For more opinions (unless HOOPSREF chimes in here) you might post/ask this on the officiating.com forum Seems to be another of those unusual things that can happen, but not specifically appear in a rulebook.