This is a sure-fire way to damage your hearing. You don't realize how loud the sound it cranked up until you're going slow or stopped. Even then the brain attenuates what you hear to some degree but the sound pressure level stays the same.With my SENA speakers I would have to crank it all the way up and still couldn't hear the music too well as speed.
Your method or earplugs and helmet speakers minimize or eliminate damage. Custom molds around good earbuds afford the best solution but as you say are pricy. And some hearing problems like auditory nerve damage can't be fixed. But it can be prevented.