just another ref wroteThis does not relate to the OP, where a buzzer sounds after a made free throw. At this point, it doesn't matter when the sub arrived at the X.
The horn should not be sounded until after the substitute has reported, so why would the "buzzer" sound after a made FT unless a substitute is waiting at the X? If there's no one there, then we keep going. That was the point that BillyM was making.
Take a look at 3-3-1d.
If entry is at any time other than between quarters, and a substitute who is
entitled and ready to enter reports to the scorer, the scorer shall use a
sounding device or game horn, if, or as soon as, the ball is dead and the
clock is stopped.