Crackles & death sounds like bad ground.
Undo & redo all ground connections, preferably cleaning all surfaces (wire brush, eraser, contact cleaner, whatever). Assume all metal-metal contacts have insulation - scrape/clean it off. (Bolts/screws included.)
That could also be the unstable voltage situation, and that suggests a bad alternator-chassis to engine to chassis/body ground, but bad body (or engine) to battery- ground could also account for it. So maybe that too is the audio ground problem.
For the audio, it could also be bad power. You could clean all +12V contacts etc but try swapping the fuse first (in case of thermal fracture, or because it is a glass fuse) else at least remove & refit (to scrape the contacts).
Although unstable voltage could be a bad regulator or ground, and the audio problem +12V or ground, I'm wondering if I should bet with 95% confidence on poor grounds knowing that mechanics and service centers rarely find such a basic problem (despite it being one of the first to be eliminated), and that the laptop was too stupid or primitive to detect ground problems because the aforementioned usually specify such programming etc. (But was the elec asked why, or was it merely a report?)
I hope I lose the bet....