How this thing detecting a gun shot put the driver in the frame for doing it is somewhat of a puzzle to me.
That is the easy part. If you have CCTV that is good enough to see a person getting in the car, or even people around the car, you can make the statements
a) There were only person A and person B in the car at the time
b) Due to the position and angle of the wounds person A could not have shot themselves
c) Our amazing microphone system placed the gunshot at this place and time, which happens to be where the car is.
It's all circumstantial and it's quite the house of cards. If they are all correct everything is fine. When you have documents which routinely has people massaging one or more of the facts then it all falls over like the house of cards it is.