I'll try to shed some light here. If I use the fiber optic audio port, then plug my headphones into the jack on my tv, volume is plenty loud. But, now I have a wire connected, and I can no longer use any voice chat features.
If I just use my wireless headphones with the usb dongle, as intended - and how I use them for every other game I own, the overall volume is just not loud enough. Even though all adjustments are maxed out. Voice chat is very loud, but car's noises are just kinda muted. It's like the car volume slider needs to go to 150. PCars 1 didn't have this issue. (PS4 user)