A few years ago I bought a bluetooth dongle off e-bay after reading a review on ADV rider. Google BT-019 bluetooth.
The cost is about $15, it's re-chargeable, has FM built in, and you can just jam the belt clip in-between the helmet foam and the helmet shell. I leave mine on my helmet all the time. You can also plug into it with a headphone plug if you don't want to do bluetooth such as on longer rides where using bluetooth will really drain the battery on the sending device unless it is hard-wired to your bike's battery.
Hearing anything at speed is indeed the problem. Sometimes I use the Koss foam earplugs speakers, but I tend to just plan on listening only when going off road or on dirt roads where the speed is slower. In that case I wear regular earplugs and use speakers I installed in my helmet and crank up the volume.
Ride while your body will still let you.