why does a pot of boiling water quiet down after it reaches boiling point but is still roiling?
Oh, you've come to the right place.  Many years ago I asked this question as a kid on a TV write-in programme called Don't Ask Me.  The TV people invited me on the show and built a series of demonstrations to explain.

The first thing that happens when you warm up a cold pot of water is that the pan bottom gets to boiling temperature.  The water starts to boil.  Liquids boil when their vapour pressure equals atmospheric pressure.  Otherwise bubbles could not form in the the liquid.  So, the pan bottom gets hot, bubbles form, but then they move up into cooler water and implode as the vapour pressure drops.  This makes a sharp snapping sound.  Lots and lots of these imploding bubbles make a roaring sound.

As convection currents then start to mix up the water, it becomes more uniform in temperature, and the bubbles actually rise to the top of the liquid and burst.  The implosions stop and you get the softer bubbling sound.

Hope this answers your question!