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I'd ride with iPod sometimes on the street (and track ) and I found the earbuds would move too much due to the helmet being buffered by the wind. 30 minutes in the earbud was jammed sideways in my ear canal and all I was getting was Major Pain.
So then I rigged it so the earbud wasn't 'in' my ear but more like nestled in the helmets ear cavity. Thus I'd still 'hear' the music but without it smashing into my brain. However at higher speeds the wind noise was so much that it'd basically drown out the music anyway.
Have decided it wasn't really worth the hassle at that point.
Note however that I still could hear bikes, cars, pavement, kneepucks, etc all around me. Even with music cranked up.
Now I just listen to iPod while riding Motocross. That's fun.