You also had speakers in your helmet, blasting tunes out and used as phone. I route I personally would not go down, as speakers are over powering surroundings.
And have happily held phone conversations at *cough* speed. So whilst I can't condone phone usage whilst riding, it does show that the helmet kept the wind noise out

. Also, not that you'd fit my helmet (ooo-err missus), but you'd be surprised how quiet it was all set.
That said, I really do need to try new lids. I rode back from shop without ear plugs, even 30-40mph the wind noise was considerable, 70mph would be unbearable. Sometimes on the commute the plugs have slipped out, my ear would have slight ringing when I got home, so way too loud.
I used to use plugs with my previous helmet (just the yellow disposable ones), and:
a) found them to be a little too effective
b) prone to falling out (not completely, just shifting enough to allow too much noise in), maybe I have funny ears

c) can confirm that a properly fitted helmet does really make a difference