I was trying find out if it was legal to use earplugs while riding and came across this lovely site:
Still trying to dig into the local laws as to usage here in WA. Helmet mounted speakers too. I will continue to use my earplugs no matter what I find as I've not had a problem hearing sirens, horns and loud piped bikes in 20 years of use.
With plugs properly inserted there should be no discernable difference in noise level between holding your hands over your ears and with hands away from the ears. At least that's what the WISHA guy told me while I was getting certified to do respirator fit tests a few years back.