Acoustic Tests at Stonehenge
This picture shows clearly how the stones are shaped to focus sound. Whether intentional or not, this would have the effect of focusing sound. Imagine finding glass from a giant telescope, and claiming it was accidentally shaped to focus images! Sarsen is so difficult to shape that it seems extremely unlikely that this is accidental. All the stones are shaped and point in the same direction, so this is not just a randomly or naturally curved shape.
This one is not as curved.
Aaron Watson was the first to point out that the stones were curved not straight. Sarsen is incredibly tough, and Stonehenge is well known for the care with which the stone was dressed, so it seems unlikely that any shapes in the monument could be accidental, especially when all the stones are shaped this way. This National Geographic clip discusses how the stone was polished to be like marble, and carefully shaped.
This one however shows the sarsen stones at the entrance to the site, obviously curved.
As is this one.