Wow - lots of questions. I will try to address.
1. Sound on camera.
The camera has a microphone, but I don't think I would want to use it with the camera on the front of the car. The A/V input of the camera has a connection for audio, but I think it does not expect mic level, but rather something like line out on a stereo. I tried hooking a powered mic to the line, and it didn't work. The external mic jack does not work while in A/V input mode. So, I have not figured out how to get quality audio through an external mic using this. Thus the need for music
2. How its mounted.
I sort of explained. I made a bracket that attaches the camera to the tow hook. Previously, I had used duct tape. I run the wire into the hood in front, and out the hood at the back corner on the passenger side, then I tape it down over the A-pillar. Then I attach it to the camcorder which is sitting in its normal camera mount. A cleaner solution would be to run the wire through the wire grommet in the "firewall" or whatever it's called in front.
3. Why did I go off.
I've got no excuse other than a lapse of concentration, and not getting the car over fast enough. I don't think it was entry angle. I just didn't turn fast enough around the tires. I realized that I was going to put a wheel in the grass, and decided that rather than try to pull it back onto the track and risk being flung inside that I would just keep it straight and drive off. I remember that there is a lot of room there to go off, but that it was wet, and when your brakes are not working too well in the grass, you question the distance to the wall in the background about 100 times before the car comes under control.
4. 156 on GPS
It is true that I had a reading of 156 max speed on the GPS on one run session, however, in subsequent runs, I didn't see anything close to that. As Jad was saying, high 130's to low 140's was more typical. I think it may have been a glitch if that's possible. One thing is for sure, there was no other opportunity for the GPS to be going that fast. So, it was either a glitch, or a fluke run. That said, I trust the GPS over any speedometer. My speedometer was reading 10mph faster than the GPS last time I looked at it.