These forums always amaze me. I would never have thought to ask this question even though on occasion wind noise is a definate saftey issue. This high background noise takes away one of the
methods knowing when a car is approaching from the rear.
Personaly I would never wear ear plugs or portable stereo equipment while riding unless there was no chance of being overtaken from the rear.
As reccomended at
http://www.slipstreamz.com I have re-routed my helmet straps so I can
put a little twist in the strap that goes in front of the ear. Whats nice is that if it doesn't work
no harm. But the concept is great. put s little wind spoiler in front of the ear to reduce velocity.
It may take a while before the winds pick up and test this.