This is the kind of thing I would suggest, it’s designed to be soundproofing, fire retardant home insulation. This is literally the exact thing you would put in-between your joists before drywall if you were trying to insulate an interior wall for sound.
There’s a product called green fiber that is a blown-in cellulose insulation that’s been treated with fire retardant you could use if the walls have already been drywalled, I’ve used that in all of the interior walls of my home when I was remodeling to try and deaden sound and have added privacy throughout the home. My basement, which I turned into a mother-in-law apartment, is so private I can’t hear my basement neighbor when they have their video games blasting through a surround sound stereo set with a subwoofer. I can feel the floor shake but, unless I am standing right next to a vent, I can’t hear anything.
Played in the creek (pronounced crik), caught pollywogs and toads and snakes. Walked barefoot to the farm down the street to buy sweet corn. Heard stories about my uncles finding dynamite in a cave near the railroad and bringing it home and passing it around at school before the fire department confiscated it all as well as stories of my great grandma holding my great grandpa at shotgun point till he did the chore he’d said he’d get to months before. I remember the internet screaming at me when I picked up the home phone. My dad’s first cell phone was a Nokia brick. The first Galaxy smart phone came out the year I graduated highschool.