Popping into this conversation almost 4 years later, but just wanted to say that I was having exactly the same problem as OP, and following Johnr777 advice, I ran into exactly the same issues. Using CSP v0.1.80-preview218.
There wasn't an .ini file for the car mod in \extension\config\car\mods or any of its subfolders. Just as with OP, creating an .ini file in \extension\config\car\mods with the copy pasted [AUDIO_VOLUME] section had no effect.
I solved the problem by moving the .ini file to \extension\config\car\mods\loaded. Then it worked as advertised. So in case anyone is looking for a solution and finds this thread, try moving the .ini to the \loaded subfolder in case \extension\config\car\mods doesn't work.
By the way, the problematic car in my case was a stock street spec Renault Clio Mk1 16V 1993 by AssettoPedia Team - mod named "ap_renault_clio_16_V" - that has ridiculously loud engine sounds both internal and external. In my case I had to configure ENGINE_INT = 0.15 and ENGINE_EXT = 0.10 to have a sane engine volume.