On their website they say that the speed of sound would be about 690mph at the height that he was expected to reach it, not 768mph – that speed just applies at

On their website they say that the speed of sound would be about 690mph at the height that he was expected to reach it, not 768mph – that speed just applies at standard temperature and pressure. I would very much like confirmation on that point, but it seems to me that pointing to the 729mph you saw is no such confirmation.