You could try an external microphone for the sound:
[ame]http://www.amazon.com/Olympus-145037-ME-51S-Stereo-Microphone/dp/B000BTAH62?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage[/ame]
I have one but haven't had chance to try it out yet. Just be sure to get the correct cable to connect it. A lot of the cheap ones on amazon won't work on the newer GoPros (Hero4). I ended up getting the one from GoPro
GoPro - 3.5mm Mic Adapter - Connect a 3.5mm microphone to your GoPro
I have a handlebar mount that will mount on the bar I have for the harness belts. It will mount between the seats.
[ame]http://www.amazon.com/Fotasy-HEROBMU-Aluminum-Handlebar-Cleaning/dp/B00JCJVID8?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage[/ame]
I am also going to mount a cheaper GoPro in the nose to get a low road shot to cut into videos. This should give the true sense of speed. I also have a third camera with a vacuum mount that I can move around the car.
I have also found a good android app (free) to capture the speed, map and gforce data. I have tested it by just hand holding the GoPro in my usual car. It looks to work great.
https://play.google.com/store/apps/details?id=com.hptuners.trackaddict&hl=en
The car is currently having some work done so I haven't had chance to try everything out and post any results yet