It is perfectly plausible that Stonehenge was arranged acoustically. There is lots of evidence of underground cave like structures nearby where the reflective acoustics would have already been toyed around with. What is the first thing people do when they enter a cave? They start making sounds and taking stock of the unusual acoustics. Any culture that inhabits caves or spends time in them ritually would have an appreciation of acoustics. I would imagine that most neolithic cultures played around with acoustics extensively and knew some basic principles.