Exhaust systems are mostly tubes that contain sound waves. (The waves produced by the pulsing of the engine combustion.)
The exhaust "muffler" works to muffle the sound by forcing it to slow down and expand before it can exit. The stock system does this with physical interruptions to the path, forcing the gases to pass through restrictive porting (the small holes) and/or sound-absorbing fiberglass matting. (FWIW, the fiberglass is simply a non-combustible material...steel wool would do the same if equal density, but steel is prone to rust, so....)
If you want to restore the sound deadening to your muffler, I'd recommend a restrictor that turned the end of the muffler into a solid tube that extended into the muffler a ways, with NO HOLES, forcing the gasses around it to enter turbulent flow and interfere with the gases allowed to pass "straight through" at the narrower diameter.
A longer slip on would be quieter, as would one with less direct passage of gasses, as would more dense packing (to a degree....past a point, it's WORSE, acting like a straight pipe once it's all packed in and clogged.)
YMMV. Hope that helps. Think about what the gas is trying to do and hinder its' progress.
