Yitro - Sh'vi'i
I'm Not Doing THAT Again!
The people saw the sounds (you didn't misread that: they saw the sounds!) as well as the fire and everything else that was going on. They were terrified that the experience would literally be the death of them, so they told Moshe, "You know what? From now on, why don't you just talk to G-d and get the mitzvos for us?" (There's a difference of opinion as to when this exchange took place. It might have been after the first two commandments, when G-d switched from talking directly TO Israel to talking to Moshe ABOUT them, but it also might have occurred after the Sinai experience.) Moshe told the people not to worry, G-d had done that for their benefit, so that awe of Him would help to keep them from sin. Be that as it may, it was an experience they were not quick to replicate, so they kept their distance and let Moshe do the talking.
G-d told Moshe to tell the Jews that now that they had experienced Him, to be careful not to be led into the snare of idolatry. (We can't really grasp how tantalizing the lure of idolatry was back in those days.) G-d told the Jews to build an altar to sacrifice to Him. It could not be built with iron tools, as these were implements of war. The altar had to be approached by a ramp, rather than stairs. Stairs would cause the kohanim (priests) to "flash" a little skin, however minimal, which was unacceptably immodest in the context of the altar. (These three commands, given in the wake of the Sinai experience, correspond to the three cardinal sins - idolatry, murder and adultery - for which one must be prepared to perish rather than transgress.)