It is the only one that any witnesses have reported hearing. I've been wondering why no one at the school has said anything about hearing a gun shot. Lots of stuff being kept close to the vest on this case. The 223 is a "small game" load. Max. distance of about 400 yds. but, not very accurate at that distance. At 100 to 150 yds. acuracy is is fairly good.
If the shooter is hundreds of meters away and using a subsonic round, you actually won't hear anything. I saw a videotape of a demonstration of this. The shooter was at some considerable distance (don't recall exactly how much) and the camera showed the target bucket getting hit. The only sound was that emitted by the bucket as it was pierced.