"And all the people saw": the sounds of sounds and the flames of flames. How many sounds were there and how many flames were there? The intent is that each heard according to his power (to absorb what he experienced), viz. (Psalms 29:4) "the voice of the L–rd in power, the voice of the L–rd in majesty."
All the People Saw the Sounds of Sounds
Mekhilta Tractate Bachodesh 9:2