trying to think of a way to crush coffee beans quietly with a hammer on the floor in a plastic bag ,

Put ear plugs in 😬😁

You can do that ....them place them in a ceramic bowl, and use the bottom of a cup/glass/small bowl to grind them.
A rolling pin, or jar can be used to grind them down also once they've been cracked .

Do you have a heavy rolling pin? I've crushed coffee beans like that before.

Coffee in plastic bag...then Wrap the plastic bag in a towel..

Go bonkers!

Sometimes I put them in a paper bag and hammer it with my mallet or a rolling pin lol

Put them on something hard and between towels