I have an older GoPro and have always been unhappy with the sound - too much wind, not enough engine. Not being able to go for the newest with the external mic, I made some mods to the case for the current one that seem to allow better sound.
A few folks share stories and pics of how theirs are mounted so I'll add those as well.
I used the standard, adhesive backed mount on the side of my roll bar (1980 Lola T-540). Then I secured that with crossed zip ties to be sure. On the corner of the camera that would touch the roll bar I added some soft but dense cushioning - kinda like the rubbery foam you would find covering the copper pipe on your home heat pump. I used a small bungee loop to hold the camera tight to the bar, with the cushion to keep the vibration down. The position and angle give me the road ahead, my steering wheel with the tach visible (sometimes) through the wheel, and the gearshift hand.
I modified the housing to get better sound by drilling a hole where the microphone is, then putting a bit of cotton ball (just a piece) in that hole, sort of like the wind mic covers you see the news people using on TV. I also ground the plastic clip just a little to allow more sound in through the side. Allowing more sound waves in through an opening, even with some cotton to block the wind, seems to work pretty well for me.
Photos below (hopefully) and a link below with a couple of laps to illustrate the sound.
http://youtu.be/h6TQWXk_se0
Hope this helps a few folks.
David