Ok so since I'm like, a musician, and I play every night with in-ear monitors, I think I can comment with a pretty decent amount of experience on this!
IMO - you are going to have volume and/or blown speaker issues if you double up with earplugs, then speakers. It's like wearing a wetsuit so you can take a scalding hot shower. Essentially, you're blocking over 50% of the sound coming to your ears, then cranking up the speakers to hear them over the plugs. You need a lot of power, and usually, higher wattage speakers than what is going to fit properly inside a helmet.
I use M-Audio IE-10 In Ear monitors - both for stage and riding. They are earphones that fit up into your canal, and have a bunch of interchageable tips, from expanding foam (what I use) to silicon flanged tips. I find that the expanding foam ones act as earplugs for the helmet by sealing my ear canal, and I can keep the volume on the music reasonable since I'm filtering out the wind noise/roar.
The best part is, they are only $100 at Guitar Center or online.
These things rock - comfy, they stay in your ears and the interchangeable tips/sleeves allow you to fit the plug into your ear without having custom molds made. And they sound GREAT. You can plug them right into an amp or directly into an MP3/Sat radio unit and get enough power to hear them well.
Good luck with whatever you decide to do!! I know I tried about 5 different systems before I settled on these.