While I haven't used earplugs, I have used a few sets of ear phones that do very well in suppressing some of the engine music only they move and can fall out easily mid ride.
Has anyone tried these?? MyEers
I have also found that using a neck warmer pulled over my head before fitting my helmet has made a huge difference in long distance riding. Not only is the wind noise suppressed but the helmet fits incredibly snug and prevents sweat and oils from getting into the helmet lining and just throw in the wash after each ride.

Still protects the neck and throat from bugs and bees etc and I haven't found it to be too warm in our hot climate.
Matt.