I'd always understood it that 'crack and thump' would only come from a round travelling faster than the speed of sound - the crack being the sonic boom as it passes overhead (hopefully) and the thump being the sound of the round being fired, travelling somewhat slower at just the speed of sound? I suppose also that flash and bang may be the difference between speed of light and speed of sound?
Well, as a ickle cadet on Kingsbury ranges, that was what a big scary man shouted at us (he was RA and preferred his ears unadorned with the Products of Peltor) and it's stuck with me ever since.
I may have misinformed a significant number of people over the years...