Insulation keeps it quiet. Usually a good idea to check the muffler every 3+ years, as heat destroys it. Usually muffler Insulation is a type of fiberglass packing. I know that Rocky Mountain ATV used to sell it. Possibly Motosport.com too. Either should have it.
On the trail mufflers I’ve done you always lose a few chunks of the insulation mat When removing the cat. Ive just put back the mat and welded it up. None any louder then another. The heat you lose by removing the cat is very noticeable Though.
Did you cut the muffler open to remove the cat? If so you could probably replace the packing, if you just broke the cat up and removed the pieces without opening the muffler I would think it would be very hard to replace.