Water degrades the fiberglass packing that's inside the muffler.This in turn lowers the packing's ability to muffle the sound.People sometimes do this intentionally to get a louder sounding exhaust.Unfortunately there is no turning back.Once the fiberglass is destroyed,the muffler has to be repacked to lower exhaust DB's.This is why they sell exhaust plugs for bikes.You plug the exhaust to prevent water from getting in during a wash.
