I don't think the NRA rule on this has anything to do with noise level or hearing protection. Even if they were legal in competition, not everyone would be using them so you'd still need plugs or muffs. My guess is that the rule is based on the recoil reduction provided by suppressors. They give an advantage in that regard to those using them, so they are prohibited, same with muzzle brakes. Whether they increase or decrease accuracy is irrelevant, I don't know of any rules that use that as a criteria. If you want less recoil, you need to make adjustments to your rifle or load that conform to the rules.