I think it depends on your helmet, and also the design of the mic coupled with the direction it's facing.
Most cheap mics will be a cardioid pick up pattern. So it will pick up most sound right in front of the diaphragm in the capsule and reject noise to the rear and sides.
So if your mic is pointing slightly more out towards the shell of the helmet than your mouth, it will pick up the resonance/drumming that the wind buffeting the helmet makes. Also, the closer the mic is to the shell, the more wind noise it will pick up.
Some mics will be omnidirectional or 'figure-8' mics which means they'll pretty much pick up song from all directions. Great for ambience. Rubbish for avoiding wind noise!!
The characteristics of the helmet are also incredibly important and just as varied. Some whistle, some roar. Depends how well designed they are. Not forgetting as well that the thickness and material of cheek pads is different from one lid to the next. And a dense/thick cheek pad will filter out a bit more low frequency sound than a thin/light pad, which will only take off a little treble.
My overall point is, it doesn't work at any speed, for every body. If you've found the perfect setup that's great! Sadly the rest of us are playing catch up with the equipment we've got! And I can't afford a new ANYTHING right now!!
My helmet is noisy at pretty much every speed. The cheek pad is too thin, and the helmet too loud to really get any better. But I bought it before I started trying to record my voice while I ride!! The next one will be very different!!
Incidentally, for anybody in Germany, my local helmet shop 'Louis' actually has a fake bike sat in front of a MASSIVE fan so you can actually see just how the lid will perform before you buy it!!